I went to a Kingswood activity centre last year with my class of 9-10 year olds. One of the activities was quad-biking (they were SO excited!)
We got to the quad bike track - they were allowed one lap of a little flat, sandy, fenced in track, children in a helmet obviously, the bike went at about 5mph, WITH AN INSTRUCTOR RUNNING ALONGSIDE holding a string attached to the quad bike meaning they could cut the engine at any point.
Fun much? After the first couple had a go, the rest of the children weren't as enthusiastic about it any more!