A comfortable tread-to-riser ratio for a six-year-old
Stairs are well regulated, with lots of rules dictating width (based on how many occupants may need to evacuate in an emergency), handrails, guardrails, nosing size and profile, and the tread-to-riser ratio (based on the standard size and shape of people). People aren’t standard sizes and shapes, although lots of design treats people that way. So for me the importance here is not how big a six year old is, or what range of sizes and shapes do six year olds come in (and how many different sizes and shapes of stairs can accommodate them). The lesson is that any design has an unintended audience, and that designer must balance efforts to focus on the intended audience and efforts to accommodate as broad an audience as possible.