Previous studies on doctor-parent-child communication at the general practitioner's surgery showed that the GP and the parent differ fundamentally in the way they enable or constrain child participation. The question how to explain these differences is at the core of the present study. The aim is to describe how the three participants display their orientation to their institutional roles and identities; how they collaboratively co-construct the course of action; and how these discursive constructions structure the ongoing interaction. A qualitative analysis of 106 videos shows that although GP and parent initially show incongruent orientations toward child participation, in the further course of the encounter all three participants jointly establish a situation in which child participation appears to be rather an exception. It is concluded that parental speaking for the child is, in a way, institutionally co-constructed; parents take their responsibility, which is hardly ever questioned by children, and GPs ratify this behaviour by refraining from meta-communicative comments and by aligning with the parent in the course of the interaction. The results are discussed in terms of enabling child participation and implications for medical practice.