What R/C car ?

[Mark V8]

Nothing TOO powerful for a six year old - yes, its for him but rugged and easy to use.
Any suggestions ?

[The_Jackal]

The Losi Mini T or Micro T Desert truck are quite good Ready to Run Packages, of course you may have to open it and check it works properly first lol

[Bungleaio]

Are you looking for toy or hobby grade?

[Mikey G]

The Maverick Atom XB (buggy) or XT (truggy/truck) 1:18th electric cars are an absolute bargain and pretty powerful for small cars. Will cost around £65-£70 depending where you go. My friends 4 year old uses one around the garden and shop and has given it some real stick with only minor breakages.
Next step up will be a HPI E-Firestorm 1:10th scale which is an electric buggy/truggy but may be a little too fast for a 6 year old to begin with as it comes with a higher powered motor. A step down to a 27 turn motor will slow it down enough to get used to it though. Prices are around £140 for this setup.

Either of these should keep you a six year old happy for some time