We're raking over some old(er) coals, but the idea of a 200 with controls and suspension sized for people of smaller stature is a winner in my book.
You could offer it with the 45mm Zokes shortened a couple inches, a less expensive shock and with levers either adjustable or simply closer to the bar. The body work and majority of the bike is "just like dad's" so the issue of dissimilar parts is minimized, and Junior has a reason to want to ride the bike.
The only place the 125-based bike might be better is weight, and with lowered suspension that might not even be an issue. If you built if off the 200 you could e-start it too ...