I got my 6 year old one of these for his birthday,
http://www.smddirect.co.uk/120411-s1...kit-versio-885
They come as a kit or built but not RTR. I grabbed a motor, speedo and lipo from Hobbyking and a cheap wheel tranny and he loves it.
He built it himself with only minimal help from me and enjoys bashing it around. He doesn't have the control for racing yet but its coming. He puts some wheels on the decking and drives round them.
Anyway they are cheap as chips and being really durable makes it ideal for any young child, or at least that's my experience of it.