Crucially, the first six years of life lay the ground for children’s cognitive, physical, linguistic, and social-emotional development. Early childhood experiences in the home and in extra-familial settings can shape the foundations of subsequent developmental stages [1]. Given that both innate and environmental factors play a role in development, adults can foster abilities and competencies with the potential to influence children’s mental health and habits over time. While parents bear much of the responsibility for this process and can impact their child’s adjustment [2,3], it must be recognized that teachers and educators can also influence children’s overall development. Currently, almost a third of European children under 3 years of age are enrolled in formal childcare [4] while 93% of children over the age of 3 attend early childhood education and care services [5]. Thus, for the majority of children, preschool is the first educational service to be accessed outside the home, making educational facilities ideal venues for supporting children’s acquisition of life skills.

Among such skills, social-emotional competencies have been shown to protect against mental health issues [6,7,8,9], highlighting the importance of early social-emotional learning (SEL). SEL is defined as the process through which individuals acquire the knowledge, skills, and attitudes implicated in five inter-related social-emotional competencies: self-awareness, self-management, social awareness, relationship skills, and responsible decision-making [10,11]. These five social-emotional competencies retain their core status across the life span, but their content changes as a function of age-related developmental tasks [12].

Self-awareness includes the ability to accurately recognize one’s own internal states, strengths, and weaknesses and how these can influence one’s behaviors. After the age of 1 year, children begin—amongst other self-aware behaviors—to name themselves (using the proper noun “I” or the terms “mine”, “me”, “myself”), recognize themselves (e.g., in the mirror or in photographs), and express their internal states [13,14]. During the preschool years, they display additional signs of self-awareness, including increasing ability to label their more complex feelings, to identify both internal and external causes for their emotions, to describe their interests and what they are good and not good at, and to reflect on the reasons why they act in a certain way, all thanks to the ongoing parallel development of their linguistic and social cognition skills [15,16,17].

Another competence that largely concerns the individual is self-management, which encompasses the ability to efficiently manage one’s own emotions, thoughts, and behaviors, as well as to set and act to achieve goals. Mastering these abilities makes it possible to control automatic and inappropriate responses, which are common during the first years of life. Children early acquire self-regulation strategies (e.g., thumb sucking, hiding, turning away in situations where they do not feel at ease), but in most cases they need help from adults to compensate for their as-yet basic abilities [18]. During the preschool years, they gradually acquire increasingly complex strategies (such as explaining their own desires, goals, or needs, engaging with others’ points of view and feelings during conflicts, being persistent in completing difficult tasks, distraction) for managing stressful situations by themselves and controlling their reactions when interacting with peers. Initially, children may struggle to implement these strategies successfully and thus continue to require co-regulation from their caregivers in order to calm down, achieve pre-defined goals, or adapt their behaviors [19,20].

Another social-emotional competence is responsible decision-making, which includes the ability to realistically evaluate the consequences of one’s choices for self and others. It implies the application of moral and ethical principles when deciding how to act in everyday life situations. For example, a preschooler might judge that stealing a toy from another child would be unfair and could lead to a fight, and thus choose to seek out another toy or object; or, they could reprimand a peer at the playground for climbing backwards up the slide rather than taking the ladder, pointing out that this is dangerous, or unfair to the other children queuing to use the slide. These examples reflect preschoolers’ understanding of social norms, values, shared commitments within a community, and reciprocal respect, which helps them to successfully adjust to their social environment. In the early years of life, children internalize the rules of adults about what is right and wrong and act accordingly, but from 3 years onwards they demonstrate a complex understanding of their own and actively enforce social norms. For example, they correct one another’s behavior, both during games with explicit rules and during pretend play [21,22]. Furthermore, they protest when a third party’s property rights are violated [23]. Thus, children enforce social norms whether or not they are directly involved in an interaction, implying an early tendency to act appropriately in social situations with a view to maintaining the integrity and wellbeing of their social group [24].

To morally judge another individual and evaluate the consequences of that person’s actions, the child needs to acquire and integrate information about what other people think, desire, want to achieve, etc. [25]. Understanding the minds of others is a prerequisite for social awareness, which is defined as the ability to adopt the perspectives of others and be empathetic toward others, including those with different personal and socio-cultural backgrounds. These capacities develop very early [26], and by the preschool years, the child has acquired the ability to explicitly reflect on the actions of others and to explain the behavior of interlocutors by inferring their feelings, thoughts, beliefs, perceptions, etc. [27].

By adopting others’ perspectives and experiencing others’ mental states, children further enhance their relationship skills, which encompass the tendency to create, maintain, and repair positive relationships. Indeed, they gradually acquire the ability to effectively communicate, listen to others, solve conflicts, cooperate with others, and offer and request help as appropriate. Having a wide variety of interactions both at home and in extra-familial contexts helps children to experience different kinds of relationships, both with peers and adults, and to identify effective strategies. For example, the child will learn to negotiate, identify a compromise, or apologize in order to solve conflicts with siblings or peers. They will learn to offer help or comfort to someone who is in distress (e.g., by drawing, hugging, or listening to an interlocutor). Thanks to attitudes and behaviors such as these, children are perceived as friendly, and are more likely to be popular with their peers and less likely to suffer peer rejection [28].

Although the five social-emotional competencies just outlined are acquired spontaneously, they are also malleable and susceptible to improvement by means of early intervention [29,30,31]. Children generally spend a significant amount of time at school, while preschool educational settings allow teachers more freedom to integrate SEL practices and activities into their daily routine. There is evidence that early interventions are more effective in the promotion of social-emotional competencies compared to those carried out with older students [32]. Furthermore, as illustrated in the next section, participation in SEL programs produces long-lasting benefits, including the prevention of negative psychological and behavioral outcomes [33].

There is evidence that children’s mental health—a multidimensional construct that encompasses multiple aspects of psychological and social functioning [34,35]—is associated with their development of social-emotional competencies. Indeed, more advanced social-emotional competencies are associated with enhanced positive outcomes (e.g., prosocial behaviors) and also help to prevent problematic behaviors such as internalizing (e.g., anxiety, social withdrawal) and externalizing (e.g., aggression, hyperactivity) issues. For example, numerous studies have identified positive associations between children’s social awareness and their propensity to engage in prosocial conducts such as sharing, helping, comforting, mediating, and cooperating [36,37,38,39]. Self-management also has been associated with positive behavioral outcomes [40,41]. Gender-related differences have been reported, with girls obtaining higher ratings for both SEL competencies and prosocial behaviors compared to boys [38,40]. With regard to problem behaviors in preschoolers, children’s SEL skills have been reported to be negatively associated with both internalizing and externalizing problems [42,43,44]. In this relationship, gender may play a role. Maguire et al. identified similar overall patterns for boys and girls (aged 4–6 years) in terms of the association between emotional competencies and problem behaviors, but poorer emotion understanding was associated with more externalizing behaviors in boys and more internalizing behaviors in girls. Furthermore, males engaged in fewer prosocial and more externalizing behaviors than did females [45].

Further evidence for the association between social-emotional competencies and mental health has been outlined in studies on the impact of SEL programs and other forms of intervention focused on emotion [31,46,47]. Meta-analyses and systematic reviews have usefully collated and reinforced the conclusions of primary studies, showing that SEL programs significantly increased positive social behaviors and reduced both internalizing and externalizing problems [8,48,49,50], with greater effects in younger students [32,51], thus adding to the evidence that social-emotional competencies protect against the onset of mental health issues.

An extensive body of research suggests that preschoolers’ mental health is linked with several early learning outcomes, both concurrently and predictively over time [52]. Children who start kindergarten with greater competence in managing their own and others’ emotions, and the ability to establish and maintain healthy relationships with peers and adults, display successful early school adjustment [53,54,55,56]. Specifically, preschoolers’ social-emotional competencies are strongly associated with their level of school readiness (e.g., literacy and numeracy skills), even after controlling for cognitive ability and family background [57,58,59]. Conversely, preschoolers who lack developmentally appropriate social-emotional competencies tend to participate less in classroom activities, displaying poorer motivation and greater difficulty performing early academic tasks [60,61]. Notably, boys tend to score more poorly on assessments of these behaviors and attitudes at school [62]. Deficits may persist across the elementary and secondary years, increasing the risk of subsequent school dropout [63,64,65]. In general, studies such as those outlined have suggested that early intervention in support of children’s mental health is valuable because it not only enhances students’ wellbeing but also boosts their levels of achievement in the short and long term [66,67].

Over the last three decades, an increasing number of evidence-based mental health programs for students—a type of intervention that was initially developed in the United States—have been conducted worldwide [34]. Mental health programs can be designed to promote different social-emotional abilities (e.g., communication, empathy) and prevent specific behaviors, difficulties, or disorders (e.g., bullying, anxiety, violence), depending on their theoretical underpinnings and specific goals, and on whether they are delivered as universal (i.e., Tier 1) or targeted (i.e., Tier 2) interventions—that is to say, whether they are for all students or only for those displaying signs of mental health difficulties [68,69,70].

Most mental health interventions for preschoolers have focused on fostering social-emotional competencies, offering children psychological, social, cultural, and physical resources for coping with stress and challenges and for building psychological wellbeing [71,72]. As such, these programs have typically been developed to enhance protective factors for mental health rather than to reduce or address existing challenging behaviors [49]. A large meta-analysis by Blewitt et al. of 63 universal interventions carried out in early childhood education and care centers showed that participation was associated with major improvements in positive proximal outcomes (e.g., social-emotional competencies), whereas smaller or no effects were observed in terms of decreased distal outcomes (e.g., challenging behaviors) [73]. The learning process necessary to display positive behaviors may require more time because the child has to integrate the new skills and adjust behaviors into everyday life. Thus, it is possible that distal outcomes may be delayed and could be more strongly appreciated after a few months, especially if parents are involved in the implementation and should change their attitudes and behaviors as well [74].

In this regard, multi-focused interventions that combine initiatives for children, teachers, and parents have showed to be effective in promoting children’s competencies and decreasing their problem behaviors [75,76]. Adults’ joint work and alignment on the best practices can enhance children’s development and the generalization of acquired skills to their everyday lives. In this perspective, principals also play a crucial role in the school setting because they can provide foundational support to the school community and reinforce the best practices. Therefore, the involvement of the whole school community should be valued in school-based interventions [77].

Additionally, as universal interventions address all students, at-risk children can benefit from the programs that are delivered at school. Most previous universal interventions on preschoolers have also evaluated the effectiveness of programs on disadvantaged children, identified with contextual criteria such as low socio-economic status and free/reduced-price lunches [32,49,50,75,76,78], belonging to ethnic minorities [49,50,76], and living in unsafe and violent neighborhoods [50]. Parental factors including education level [75], mental health issues, and inappropriate parenting practices [76] have also been considered. On the other hand, children with diagnosed mental health problems, special education needs, and disabilities have been scarcely included in the at-risk category [48,75,76], despite it being necessary to pay attention to them to minimize negative consequences, especially in the first years of life [1].

In light of these premises, further research is needed to develop and deliver universal programs adopting a whole-school approach that includes activities to both promoting preschoolers’ competencies and reducing their behavioral problems. PROMEHS was intended to fill this gap. The program was developed within the Erasmus+ Key Action 3 project entitled “Promoting Mental Health at Schools” (PROMEHS), co-funded by the European Commission. The aim of the project was to develop, implement, and evaluate a school-based, universal mental health curriculum for European students from 3 to 18 years of age. The project involved researchers, policymakers, and stakeholders from seven countries, namely Italy, Malta, Latvia, Croatia, Greece, Romania, and Portugal. The effectiveness of the PROMEHS program has already been assessed in relation to the entire international sample of teachers [79] and students from kindergarten through high school [80], Greek [81] and Portuguese [82] students from kindergarten through high school, and Romanian highschoolers [83]. The effectiveness of implementing PROMEHS with preschoolers specifically has only been documented with Latvian children [46].

The PROMEHS program offers multiple positive features, which reflect the highest, state-of-the-art standards for evidence-based interventions [84]. For example, as earlier stated, it was designed to both foster competence and reduce mental health issues. Indeed, the theoretical framework within which the program was developed covers three themes: the promotion of social-emotional competencies, the promotion of resilience, and the prevention of social, emotional, and behavioral problems [34]. This framework also envisages a whole-school approach, whereby key stakeholders in children’s mental health (i.e., teachers, school leaders, and parents) are involved in the intervention. More specifically, the program consists of a training course and supervision sessions for teachers to ensure that they assimilate the theoretical concepts informing the program and reliably implement the relative practical activities in the classroom. This means that the teachers are empowered to deliver the program firsthand, with no direct involvement on the part of the researchers. The program also targets school leaders and parents, through dedicated encounters designed to provide them with knowledge and practical strategies for promoting children’s mental health at school and at home. Involving all these parties enables a joint effort whereby all the adults with a significant role in the children’s education and care can adopt current best practices, thus facilitating the children’s acquisition of competencies and adoption of positive behaviors across different settings [49,85,86]. Another strength of PROMEHS is that the children themselves are actively involved, which further increases the program’s chances of being effective [32]. Specifically, the program entails an experiential approach to learning, such that the children are informed by their teachers about the aims of the classroom activities and are engaged in the learning process by means of self-reflection exercises, games, group discussions, and other practical activities, as illustrated in the ad hoc handbooks.