RE: Best choice for 6 year-old w/ newbie parents
babynuggo - You have just described a Traxxas stampede. The stampede or "Pede' is 2 wheel drive, water proof, tough as nails, reasonably cheap and has great parts availability when something does break. It's also tall enough to be driven just about anywhere. it also comes with a training mode so you can slow it down for indoor running.
The Pede is also a truck he can grow with. There are literally dozens of comapnies that make after market parts for these trucks so he will never be bored.
And it's only about $150 but you will need some batteries and a charger.