I'm the (adult now) child of a man who was a scuba instructor for 15 years; heck, he even did a couple of dives with Al Giddings! However, he stopped teaching by the time I was old enough to learn. If you had asked me anything about diving before I started my OW program, I wouldn't have known the answer. With that said, even children of "experienced" divers should still get proper instruction. Why do you need a driver's license? Certainly the skills can be self learned, but really would you want someone out on the roads who has no idea what the difference between the gear selector lever and the windshield wipers is? I don't want a buddy who doesn't know the difference between a BC inflator hose and the octopus. Same thing applies. With a certification card, you have a standard to judge where your dive buddy's skills are.