I think the first thing you need to decide is whether or not your child has the maturity to understand physiology and diving. There aren’t really any formal studies on the effects of young children diving, either (at least not that I’m aware of). So, if you think your child is mature enough, get him certified by a GOOD instructor. Find one that is familiar with teaching kids. After your child is certified, make sure you keep him/her within the limits of their cert. SSI has a Junior Open Water Diver for kids between 12-14 that allows them to dive to 40ft. There is gear that you can buy that’s designed for small kids - BC’s and tanks (an 80 aluminum might be too heavy).