BACKGROUND: Children's mental health is a public health priority, with 1 in 5 European children younger than 12 years having a behavioral, developmental, or psychological disorder. Mental health literacy (MHL) is a modifiable determinant of mental health, promoting psychological well-being and reducing mental health problems. Despite its significance, no interventions or scales currently exist for increasing and measuring MHL in this population. OBJECTIVE: This study has dual objectives: (1) cocreating and evaluating an intervention on children's MHL, and (2) developing and validating a scale that measures children's MHL. METHODS: Our study focuses on children aged 9-11 years attending primary school classes in various settings, including urban and rural areas, and priority education zones within a French department. Using a participatory research approach, we will conduct workshops involving children, parents, teachers, and 1 artist to cocreate an intervention comprising multiple tools (eg, a pedagogical kit and videos). This intervention will undergo initial evaluation in 4 classes through observations, interviews, and satisfaction questionnaires to assess its viability. Concurrently, the artist will collaborate with children to create the initial version of the CHILD-MHL scale, which will then be administered to 300 children. Psychometric analyses will validate the scale. Subsequently, we will conduct a cluster randomized controlled trial involving a minimum of 20 classes, using the CHILD-MHL scale scores as the primary end point to evaluate the intervention's efficacy. Additional interviews will complement this mixed methods evaluation. Both the intervention and the scale are grounded in the Child-Focused MHL model. RESULTS: The first tool of the intervention is the pedagogical kit Le Jardin du Dedans, supported by the public organization Psycom Sante Mentale Info and endorsed by UNICEF (United Nations Children's Fund) France. The second tool is a handbook by the Pan American Health Organization and the World Health Organization, which is addressed to teachers to sensitize them to children's mental health problems. The third is a 5-page supplementary leaflet produced by the nongovernmental organization The Ink Link, which teaches children the notion of MHL. Finally, we produced 56 items of the MHL Scale and listed existing education policies for children's mental health. CONCLUSIONS: After its robust evaluation, the intervention could be extended to several schools in France. The scale will be the first in the world to measure children's MHL. It will be used not only to evaluate interventions but also to provide data for decision makers to include MHL in all educational policies. Both the intervention and the scale could be translated into other languages. INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID): PRR1-10.2196/51096.< Réduire