Formal consent for children's research participation legally resides with adults, and guidelines typically recommend consulting children about their participation only from 7 years of age. How can researchers support younger children's informed decision-making about their research participation, particularly in larger-scale studies without extended researcher-participant engagement? In this paper, we share our experience of four studies (270 children) using drawings, observations, interviews, and biological and anthropometric measures and the visual information booklets we devised to support young children's informed agreement. Critically evaluating the notion that 'evolving capacities' necessarily increase with age, we consider young children's contextualised decision-making capacity. Reflecting on process and outcomes, we conclude that simple visual information booklets benefit not only children but all actors in the research process, functioning as material artefacts that remind adults as well as children of young children's right to choice about research participation.