Traxxas Slash. After seeing the punishment that kids at my local track put their Slashes through, I wouldn't buy anything else. One thing about the Slash is that the body covers the wheels which help keep suspension arms from being ripped off when they clip something.
I have a 5 year old son myself who's had several RC cars since he was 2+ (Grasshopper, Duratrax Evader ST, Associated RC10T) and nothing stands up to a slash.