I got a second hand jeep for my son when he turned two. Its fun. It can carry 140lbs so my wife can ride it LOL! Actually, I have ridden it too, but not for long.
I found a whole cd of hop ups including making it 18 volts and installing E-Maxx motors in it! I don't remember the exact website, but if you want the cd you can contact ccwbischof@charter.net Maybe a search for "Will's power wheel page" will turn up something. The fact is that the jeep is fast enough for my son at 5mph and I am not anxious to chase after him on my bike.

BTW the 18 volt conversion is simply to wire a third 6 v battery in series, the stock motors are based on 18-24 volt drill motors and can take it. It will then do about 12-15 mph.

Here it is!
http://ungn.net/email.html