Background: Digital interventions, including mobile apps represent promising means to provide effective mental health support to young people. Despite the increased availability of mental health apps, there is a significant gap for this age group, especially for younger children. Research investigating the effectiveness and development process of child mental health apps is limited, and the field faces persistent issues in relation to low user up-take and engagement, which is assumed to be a result of lacking interdisciplinary approaches. // Objective: We present the development and design process of a new mental health app for children that targets their emotion regulation abilities. We describe the creation of a new interdisciplinary development framework, to guide the design process, and explain how each activity informed different app features. // Methods: The first two stages of the framework employed a variety of methods, including: 1) classroom observations, 2) public-engagement events with the target group (N=21), 3) synthesis of the existing evidence as part of a meta-analysis, 4) a series of co-design and participatory workshops with young users (N=33), clinicians (N=7), researchers (N=12), app developers (N=1) , designers (N=2), and lastly 5) testing of the first high-tech prototype (N=15). // Results: For the interdisciplinary framework we drew on methods derived from the medical research council framework for complex interventions, the patient-clinician-framework and Druin’s cooperative inquiry. The classroom observations, public-engagement events, and synthesis of the existing evidence informed the first key pillars of the app and wireframes. Subsequently, a series of workshops shaped and reshaped the content and app features, including games, psychoeducational films, and practice modules. Based on the prototype testing sessions we made further adjustments to improve the app. // Conclusions: Although mobile apps could be highly suitable to support young people’s mental health on a wider scale, there is little guidance on how these interventions could be designed. The involvement of the different methods and especially the young users was very valuable. We hope that the interdisciplinary framework and multiple methods that we applied will be helpful to others who are also aiming to develop suitable apps for young peopl
