With the exception of a few geographic locations (e.g. Quebec), there are no laws regarding recreational scuba diving (other than laws about compressed gas storage and transportation). However, the scuba industry self-polices. Scuba rules/guidelines/recommendations are especially enforced in a training environment. They are also enforced by recreational scuba professionals, shopkeepers, equipment or air providers, and even by other divers via peer pressure.
To answer your specific question: it depends. If you did it in our shops pool, we would stop you and explain the rule and rationale. If you persisted, we would remove you because to get in our pool, you signed an agreement that you would follow scuba rules. If you did it in your home pool, the only probable legal risk would be if someone thought you were endangering your child.
However, recall that embolism is reported as possible in water that shallow. Depending on your childs age, why not have your child do a try scuba (as early as age 10) or one of the pool-only experiences offered by a couple of agencies (as early as age 8)? That way, at least it would be conducted by someone trained specifically in risks and rescue techniques.
Were it my son, Id first make sure he was a comfortable swimmer, then do snorkeling, then consider trying scuba on the surface, or snuba (as young as 4 according to the snuba website).
