First time riders feel safest with their bottom on the saddle and their feet flat on the ground. That's how a child with a 21" inseam will fit the Owl with the seat post dropped as low as possible. Once kids get more comfortable on two wheels, they'll want to sit on the saddle with the balls of their feet on the ground. Thus, a confident rider with a 19" inseam and a first-time rider with a 21" inseam may both feel comfortable with the same set-up on their Owl.
Your child will ride the Owl on trails, to school, all over the bike park and probably sleep with it in his room. (Sorry about that!)