I can imagine a fully charged standard sized aluminum tank sinking a person the size of a skinny 10-year old, even if the child wasn't wearing a weight belt, as long as he wasn't wrapped in a significant amount of neoprene. If it was a steel tank, even more so. If he was wearing a lot of rubber, then maybe not. So in terms of the speculation on the weighting, can you recall what sort of exposure protection he had on? Did you notice the size/type of tank?
In terms of prevention (and without knowing the answers to the above), when working with kids, I always use the smaller aluminum tanks, not only because they really don't need any more air than that with their small lungs, but also because of the risk of overweighting them in the water, not to mention the potential for strains and injuries out of the water from carrying the weight of a heavy tank on a small body frame.
In terms of prevention (and without knowing the answers to the above), when working with kids, I always use the smaller aluminum tanks, not only because they really don't need any more air than that with their small lungs, but also because of the risk of overweighting them in the water, not to mention the potential for strains and injuries out of the water from carrying the weight of a heavy tank on a small body frame.