Background Mental health issues in youths have cascading negative effects on school outcomes, professional life, and physical health. Psychological well-being (P-WB) is an important protective factor against mental illness. Preliminary research suggests that mindfulness- and yoga-based socio-emotional learning (SEL) interventions can each have a positive impact on preschoolers P-WB. The objective of this trial is to rigorously evaluate the effect of a 24-week combined mindfulness- and yoga- based SEL intervention in preschool children from a French socio-economically disadvantaged area. Methods The P-WB promotion intervention is compared to a wait-list control condition in a cluster randomized controlled trial (RCT). Sixty-four pre-Kindergarten classrooms are randomized to the intervention or control group. Primary outcomes measure self-management capacity and core P-WB components: connection, insight, engagement, and positive relationship. Secondary outcomes include measures of mental health, executive functioning, and school performance. Primary and secondary outcomes are assessed through teacher questionnaires, standardized observations of children in school context, and experimental tasks and by collecting results of the national evaluation at first grade. All children-level outcomes are evaluated at pre-intervention, at the end of the intervention, and 1 year later (follow-up analysis), to the exception of school performance which is evaluated at follow-up only. Intention-to-treat analyses, accounting for clustering within classes, will adopt a random effects linear regression model to examine outcomes for the intervention versus control children. Discussion This is the first trial to rigorously evaluate a combined mindfulness- and yoga-based P-WB promotion intervention, and the first RCT evaluating a SEL curriculum in French schools. Results may have key implications for P-WB promotion in preschool children. Trial registration https://www.drks.de/ DRKS00028623. Retrospectively registered on 30 May 2022

Mental health issues in children and young people are frequent and can have enduring negative consequences. Preventive early interventions delivered at school may foster psychological well-being, and preliminary evidence suggests that mindfulness-based social and emotional study was to evaluate a mindfulness-based SEL curriculum including a French adaptation of the Kindness Curriculum (KC), delivered by pre-kindergarten teachers, in a cluster randomized control trial. Sixty-four classes (761 children, aged 38-58 months) from a socio-economically disadvantaged area in France were randomly assigned to either intervention or waiting-list control conditions. Indicator -management, positive relationships with teachers and peers, emotional processing and executive functioning were collected through teacher-rated questionnaires, standardized observations, and behavioral tasks. Results in mental health, including a reduction in emotional, conduct and peer relationship problems, and a reduction in teacher-child conflicts. No significant effects were found on the other indicators. Heterogeneity analyses revealed stronger effects of the intervention on mental health and positive effects on self-management, relationships with teachers and relationships with peers in several subgroups. This program appears as a promising early intervention promoting mental health and positive relationships, especially for at-risk subgroups of preschool-aged children.