Interviews and focus groups are interaction design user research methods that aim to understand user needs and goals regarding the potential use of digital products. In the case of designing Digital Behaviour Change Interventions (DBCIs) for health, these methods need to be adjusted to allow explicit focus on understanding determinants that possibly influence individual health-related behaviours (for example, beliefs, motivations, social pressure). Behaviour change theories and models help to identify these determinants of influencing behaviours. However, interaction designers are not competent in having knowledge of behaviour change theories, which makes it difficult to integrate this knowledge into interaction design user research. It can limit the design for theory-based DBCIs. The current study proposed and evaluated a generic topic guide template for interaction design user research enriched with a behaviour change theory. The paper proposes guidelines for adapting and analysing the data. The proposed topic guide template was adapted to understand the needs and age-related differences of children with type I diabetes. Focus groups were conducted with parents and medical professionals. Results indicated various behaviour change-related needs of the children and highlighted age-related differences in children's skills, independence, and motivation to manage diabetes. Various behaviour change theory-based design implications for interaction design were derived. The results highlight the importance of enriching the user research methods with behaviour change theories and models for designing DBCIs for health.