My 3 year old boy loves his powerwheel 4 wheeler, I might be jump'en the gun a little here, but soon he'll be ready for somthing a little more powerfull. Right now being electric it keeps up with my grass cutter on full speed and he can drive better then me. The plastic wheels dont allow it to go up much of a hill, and it does'nt have enough power to go through any kind of thick grass/brush. Sooo over the winter I'm gonna build a frame for the parts I have collected and use the 4 wheeler body on it. So far I have a sweet OHC 6.5 Honda motor thats very light, a peerless transaxle (small single speed F-N-R), a front axle from a small rear engine JD and some cool little knobby tires for the back, and the hard to find 6" rim's with 3/4" keyway. For a clutch I'll run a cyntrifical and run a mini bike hand throttle.
All I had as a kid was a little go-kart with a tired 5hp flat head one wheel drive that was a slick. Went quick, but had no low end power or traction for any off road fun, but really enjoyed it anyways.
Oh, all most forgot, I'm gonna mount the transaxle upside down to keep the motor low for a safe center of gravity.
I stole the idea from george#22http://www.extrememowers.com/VideosOther.html
He has 12hp with a 3sp.