I did a lot of work with 10+ year old kids doing OW certs for a very concientious dive shop. I observed a few things about very young dive candidates:
Each kid is different. If they like it, and are comfortable in the water, I see no reason to not let them dive under strict supervision. If they don't like diving, don't force them. Find a warm water resort and snorkel until the kid is a little older.
Kids don't keep track of time, depth, NDL status and air pressure. A responsible adult must be matched to each kid to monitor thier dive. I wouldn't take a 10 year old beneath 40 feet, and I prefer 30.
The biggest problems kids have is moving around with all the gear on the surface, and operating the BCD inflator. The solution for the gear problem on the surface is to make sure that gear is well matched to the student. There are smaller BCD's, and we discovered the Dive-rite transpac could be fitted well to a child. If you have a 72 cf tank it might be easier than hauling an 80.
Intergrated weights always worked better than belts, (kids have NO hips) and it helped to walk the child to the lake, than add weights in the water.
The biggest problem in the water was simply working the inflator. Smaller and weaker hands have a hard time operating the buttons. Some kids had to use both hands on the inflator. I reccommend giving your child lots of dry land practice on the inflator so he can add or release air comfortably. Practice giving him signals to add or release air.
Equalizing was often the biggest hurdle. Plan on taking plenty of time to descend.
Kids don't give you the feedback you get from adults. They might be shy about admitting a problem to an adult. Be very observant and ask detailed questions when you feel uneasy. Not: are you OK? Try: does the BCD fit? Are your ears clear? Can you see out of the mask? Give them more time to reply.
Good news, I found that kids were the ones who did their homework before the class, and really paid attention. They are used to being in school.
I took one kid for a tour of the quarry with multiple compass headings, in green water, and he stayed next to me with perfect bouyancy control for a 25 minute tour. When they get to be 13-14 years old they will be swimming circles around you. It's fun, and I love to see any kid do something interesting.
Good Luck,
Steven