This study presents the methodology based on a co-design approach to develop ERA, the Emotion Regulation App, designed to at prevent childhood obesity. Twenty-one focus group meetings were conducted with 11 children aged 6 to 10 years. Each session had specific objectives and activities: initial sessions focused on introducing the research team, creating a friendly environment, and discussing emotions and their regulation. Subsequent meetings included co-creating the app's narrative, protagonist, secondary characters, and scenarios. Creative and generative activities, such as drawing, role-playing, and model making, were performed to express ideas and preferences. Other sessions involved evaluating musical stimuli and proposing activities for the app. The final meetings included refining feedback texts and evaluating the created material. This collaborative process enabled the customization of the application and each of its parts to the needs and preferences of younger users, enhancing potential treatment adherence and thus effectiveness in promoting appropriate emotion management techniques, and ultimately reducing obesity risk.