While I fully support active children, there is something to be said for having a basic respect for property. A good bike will stand up to a lot of use, but there will always be a limit to how much abuse a machine can take--even the best of machines. I'd suggest getting something on the low end of decent (from somewhere other than wall mart--you don't want a "toy", like MM2k said) and teaching him some basic maintenance. A kid can learn a lot from having to fix (or at least help fix) the problems he caused. chains do slip, things do come loose, and there are a lot of things that aren't too hard to adjust if you have a tool or two. The kid learns some life skills (like problem solving, and how to handle a screwdriver), discovers the value of preventative measures (like its easier to take care of it than fix unnecessary damage), and hopefully has a blast in the meantime.