Unlike
@rivers, I have had young kids totally bomb out in the pool but that was in the context of a course and in cases where the kids were put under severe pressure to be there by their parents. It turns out that a parent can totally break a kid mentally and set them up for failure before they even start on something... so sad to see.
The DSD, however, is very benign, and I wouldn't worry about that.
As for training; physically the kids I've trained have been able to dive provided they can clear their ears. Most of the time if there is trouble, it's ears. Of the ones I've trained one was very small for his age and had trouble handling the gear because it was too heavy for him but once in the water he was ok aside from the CESA. After hitting a dead end with him on the CESA I had to phone PADI and talk to a training consultant about this case because he was unable to perform the CESA to standards due to having lungs the size of a small rodent. With their help he (and I) got through it.
Most of the kids under the age of 14 that I've trained (or assisted in training) have been ok in the water because they don't fear it, which is where I believe the rub is. An inability to estimate the risk is as dangerous (in my mind) as an inability to perform a skill. I've seen kids to some exceedingly dangerous things and be completely oblivious to the fact that anything even went wrong.
When it came down to training my own kids, I took them in the pool as soon as they showed interest in it. In the case of my daughter, she really liked it and wanted to get trained from a very young age. I took her to the pool when I was working (obviously with permission) and just let her piddle around while I was doing my thing. When push came to shove and she started really pushing me to train her I told her no and made her wait until she was 14. That is the age at which I see a big change in the kids' ability to accurately estimate the risks they are taking. It think it must have to do with brain development and the ability for lateral thinking. I'm no psychologist though so I don't know, but that's what it looks like.
So I started my daughter's training at 14 and finished it when she was 15 (I was in no hurry) and then after that started thinking to myself, "if I wouldn't train my own child younger than 14 then why would I train someone else's child at a younger age?". At that point I stopped training kids under the age of 14 entirely.
That's all course related, however. A DSD is not a "course" in that sense and I'll still take kids on a DSD in the pool with no reservations whatsoever.
R..
