The T1DCoach mobile application is designed to educate patients—children with type 1 diabetes (T1D) and their caregivers and diabetes educators. The idea behind the mobile application is that its users perform actions that the patient needs to perform in real life. These include measuring blood glucose levels, operating the insulin pump, meal calculation, bolus administration, etc. These in-application activities are performed on the patient’s digital twin. To increase user engagement, gamification elements have been implemented in the application. An important element of the T1DCoach mobile application is its interface, which should be adapted to very different groups of users: children, their caregivers and educators. In addition to presenting the T1DCoach application, the paper presents the stage examining the quality of the interface using three research groups: children, their caregivers and educators. The research was conducted using the scenario method, using eye-tracking, recording activities and thinking aloud. After the application testing sessions, surveys were carried out using the System Usability Scale method and focus group interviews were conducted. The research results are presented in the article along with the most important recommendations for improving the application interface.