This study conducted formative research to assess the acceptability of lay summaries and thank you notes, as well as to refine and expand guidance on participant and family engagement in Pediatric Trials Network’s (PTN) pragmatic paediatric clinical research. Researchers conducted in-depth qualitative interviews with adolescent clinical trial participants and caregivers of paediatric participants in four trials conducted by PTN across eight sites. This is the first study to describe stakeholder preferences for thank you note content and layout. Using these findings, researchers finalized PTN’s trial communication guidance for use in future PTN trials.