Background: Interactions with children in clinical settings are often criticized because parents and medical professionals speak for children rather than to them. Such approaches do not take the agency of children into account. Objective: First, to examine how children enact agency in a clinical encounter and draw lessons from this to improve health-care practices for children and, second, to explain how looking at agency might help to move the participation agenda forwards. Design: A qualitative study incorporating a range of methods, including participant observation, interviews and focus group discussions. Setting: Three hospitals in the Netherlands. Participants: Children with diabetes type 1, between 8 and 12 years (n = 30), parents (n = 22) and medical professionals (n = 16). Results: Children do not simply accept the recurrent health education from medical professionals. Instead, they attribute their own personal meaning to their disease and treatment. Drawing from their years of experience with the disease and health care and the image of a passive and vulnerable child, they actively find ways to balance personal goals with medically defined goals. Conclusion: Efforts to facilitate child participation should be based on insights into the ways in which children enact agency in the clinical encounter. Our data show that children already participate in health care and that their enactment of agency is based on a practical logic. Understanding of children's current participation and agency is needed to more successfully attune their treatment to their daily lives with diabetes. This is crucial for the success of treatment and the well-being of children.