I'll try to make this short...
I'm for getting babies in the water long before they can swim. I'm for getting very young children into masks & fins & snorkel long before they can use them with any competence. I'm for letting children dive long before they can be competent divers. But there is a huge caveat in those statements, and it is this: Parents are responsible for their children. Parents must be parents and know what they're doing before they cut their children loose with any activity, whether it's walking, riding a bike, shooting a shotgun or diving. There are risks particular to a child's psyche, physiology and development that must be accomodated and monitored closely as they grow and gain experience.
Let me offer a specific example from my own youth. I got my first shotgun when I was six. A shotgun in the hands of a six year old?? Preposterous, you say. Dangerous, irresponsible, right? Why the dangers to others aside, just the kick from a shotgun could injure a six year old. But the way it was done was perfectly safe. I had to carry the gun with the breach open. I had no shells. After a squirrel was treed and I had him spotted I was given one shell, and under direct supervision allowed to take my one shot. I had to observe all firearms safety rules with my open-breach, empty gun at all times. As I grew older, and demonstrated sound reasoning and safety practices, and especially barrel awareness and "all guns are always loaded" behavior the supervision was relaxed until by the time I was a teen I could go hunting on my own. As a result of my upbringing I am safe, comfortable and competent with firearms - and I am appalled at the poor firearms safety practices I see in the general public, including those who have gone through "hunter/safety" courses but weren't raised around guns.
Oh, yeah, almost forgot an important element of the training... If successful, I had to clean the squirrel myself.
Diving approached in the same way can produce safe, comfortable and competent divers.
Rick