I see it in the St. Andrews Jetties from time to time with an adult and a small child in the "kiddie pool" (for those not familiar it is usually pretty good vis and about 10' deep max) I always approach them with some sound advice and have actually had one father stop what he was doing and took his very young daughters scuba gear off, not to say he didn't put it back on when I left the area. There is not much you can do other than report them to the park ranger for the sake of safety for the child. We don't always have the whole story to as what the situation could be but its never a bad idea just to strike up a conversation and not necessarily go into what they are doing.