there's a lot of cool ones out there sourdough, electric is most likely in that budget, but if your looking at gas drive couple wont get it a buddy of mine is right into that stuff and his latest is well over eight hundred. I'll ask him but i don't think I will get an answer before xmas.
Best to you and yours.

Traxxass makes some damn durable FAST products ready to run.   I stil have my TMAXX from several years ago. 

Electric or nitro they are FAST and durrable

Maybe WAY to fast for a six year old though unless you got a lot of wide open property.    The speeds these things travel will take a leg out no prob

http://traxxas.com/