yeah, that's why I integrated a seatbelt into the one I built. It's a very-Mom-thing to visualize all the things that could go wrong! Well since I posted this, I got tired of beating my head against this particular wall. While I was looking into other possibilities, her physical therapist at school mentioned that she's been riding a bike there, sort of like an adult tricycle but more compact, with all sorts of trunk braces and velcro on the pedals to hold her feet and whatnot. Totally blew me away since the motion of pedaling has been right outside her range for so long. So we hope to have one of those by mid-summer & can work on her riding the paved-rail trails around here with us. Pricey little pup, but not as much as a car, which most teenagers her age are needing
oh, I just realized, I never mentioned her age in my original post. She's 17, which totally changes the parameters of my original question, oops! And by the way, thanks so much for your great replies!