That's a pretty slick setup you've got with those bikes. My daughter (now 13) still rides her 24" single speed cruiser, but we also don't have many hills and she just rides to have some slow speed fun on the bike trails. Her bike at 9 was a very old banana seated chopper style girls bike from the late 70's that I rebuilt. It had some creaks and squeaks but she loved it with the rainbow paint and little basket up front for whatever she thought to carry that day. And I'm always surprise at how far she can ride those bikes. The first time she took the chopper out, she rode 12 miles in one go!