Childhood obesity is one of the most serious health issues of the twenty-first century, challenging worldwide public health. Although literature converges on the crucial role of self- and emotion-regulation processes in children's obesity development, intervention strategies still rely on lifestyle changes (e.g., diet, physical activity, education, and behaviors), while achieving disappointing results. However, technology-based strategies (e.g., mobile apps), coupled with a serious game approach, have been demonstrated to be valuable tools for facing childhood obesity, as they encourage children’s participation and maintain adherence to the intervention. Thus, we intend to develop a serious game app through a co-design approach, a strategic method involving end-users (children) in the design process to understand their needs and perspectives. The paper describes the theoretical framework and the Emotion Regulation App (ERA) design process for children aged between 6 and 8 years.