My son is 9 (just turned 9 in July). I have been hauling him around on my bike for most of the summer. He does pretty good. I picked up an
M2R full face helmet (MR 15 Jr). It was inexpensive, but it is DOT compliant and Snell certified. Knowing that kids grow fast, I didn't want to break the bank on a helmet I may have to replace once or twice a year. I originally got him a 3/4 face HJC helmet and goggles, but he didn't like the wind directly in his face. Currently he wears an extra heavy duty denim jacket and shoes with some ankle support. I still need to get him some really good gloves. He doesn't like to wear gloves very much. There is a big YamaHonZuki dealer that has a huge accessory department that includes all kinds of gear for kids (you could go broke real quick getting gear just for kids).
He does pretty good on longer rides now. I stared off just taking him on 15 or 20 minutes rides around the neighborhoods and worked up from there. A few weeks ago we took off for about six hours and he did great. I think the secret was that we stopped about every half hour or so for a minute or two. I can usually tell when it is time to stop, because he will start to wiggle around some (9 year olds were not created to sit still for very long), when we get going again he does great again. His grip is pretty good. Sometimes he wraps his arms around me (if its cooler out), but most of the time he clamps onto my sides.
Soren