Background <p>Physical inactivity, sedentary behaviour, and poor sleep are growing concerns among primary school children, contributing to poorer cognitive, physical, and psychosocial health. Happy Homework, previously a paper-based homework intervention, was adapted into a digital format to engage children in improving their 24-hour movement behaviours at home. This study outlines a participatory design procedure used to co-design the digital app with key stakeholders to ensure it was user-centred, age-appropriate, and scalable.</p> Methods <p>Three rounds of iterative co-design workshops were conducted with children (<i>n</i> = 11), teachers (<i>n</i> = 8), and parents (<i>n</i> = 9) who reviewed paper-based prototypes, and in the final workshop, the Happy Homework app on smartphones. Iteratively, we thematically coded the qualitative data generated by stakeholders in the co-design workshops to directly inform subsequent app design specifications.</p> Results <p>The co-design approach resulted in numerous stakeholder recommendations for software enhancements which were unforeseen by the research team. These included eighteen distinct software enhancements that led to changes in the content, navigation, incentives, and appearance of the app.</p> Conclusion <p>Stakeholder involvement was essential for identifying usability and design concerns, and for outlining solutions unforeseen by the research team. Concerns identified in initial co-design workshops did not reoccur after changes were made to the app. As such we deem the co-design process invaluable in guiding the development of the Happy Homework app ensuring its features are relevant and aligned with stakeholder input. The refined app will undergo formal pilot testing to evaluate user experience and feasibility.</p> Trial registration <p>N/A.</p>