Despite growing adoption of participatory design frameworks, children's involvement in shaping design challenges remains largely adult-driven. Problem spaces are typically predefined before children enter the process, limiting their ability to articulate their own needs and constraining the authenticity of resulting outcomes. This study examines whether child-centred design (CCD) can be understood as a child-specific expression of inclusive design (INCD), and how an integrated approach to both frameworks supports agency, play, and equity in design research practice.

Using Karen Feder's (2020) CCD framework as its primary methodological lens, this research conducted three iterative co-design workshops with children aged five to nine, using self-regulation tools as a frame of inquiry. Through organic participant attrition, the study evolved into a longitudinal case study with a single six-year-old co-designer, JC, whose sustained engagement across all three sessions produced a richly detailed record of child-led design thinking in practice.

Three primary findings emerged. First, authentic child agency operates as a continuous social negotiation rather than a static condition granted by researchers. Second, play functions as a rigorous, multimodal methodology for design thinking. Third, and most significantly, the study identifies a "Process-as-Prototype" effect: the inclusive values embedded in the research environment were directly replicated in the child's final design output. Together, these findings establish that child-centred design is not just compatible with inclusive design, but a natural child-specific expression.