I've looked at the 200 and would have started there but no local dealer and I have less into the 120 with mods and I think it's comparable. My boy turned 6 in march, and is 55lbs. My girl is 8, six inches taller, and 60lbs. The problem for her will be seat height, not so much the 120s capability. Her legs are just getting too long.
I think you would be disappointed going from a modded 120 to the 200 (purely on size and speed). The longer track is a game changer though. As are the running boards IMO.
I haven't sat on the new 400 offerings from cat but we have NO dealers for miles.
We have 96 panteras with 550 in them at the farm and both kids can "drive" them, but they can't ride them. Hell I don't want to get them stuck, they would be hooped. For my daughter, that's not much of an issue, she's a point and shoot kinda driver. But my boy is throwing his 120 around and is a beast on it. To put him on an old indy or similiar is going to suck for him. I have been looking at 300 freestyles from skidoo.