Oh dear, lets not fight team!
Starfire: I agree with your own question - why SHOULDN'T a junior diver be allowed to use Nitrox?? Yes, Nitrox is most beneficial in shallower water where entry level divers are meant to be anyway. Even if you're within your air table or computer limits, Nitrox will always be safer from an inspired nitrogen point of view as you're going to ingass less of it. This leaves you with a reduced chance of having a bend (all other factors excluded for the moment), less silent bubbles, less post dive fatigue, and in the case of children less chance of any assymptomatic decompression illness causing any long bone stunting and long term injury.
Yes, there's an increased risk of oxygen toxicity and we don't know everything about the long term effects of O2 on children, but I'd argue that in general shallow depth sports diving you'll never get anywhere near either your CNS or pulmonary O2 limits anyway, so this argument is fairly well negated. The danger is with advanced and irresponsible divers pushing their limits on longer deeper dives!
The only argument I'd concede to is the enhanced level of understanding and responsibilty that's required to pass the course, always analysing the gas, and keeping within prescribed depth and time limits (that said 36% Nitrox is safe to near 28m/90ft even at a ppO2 of 1.4, a depth which is way beyond where a junior diver should be anyway). Also junior divers should always be diving with a parent or guardian anyway, so where's the problem (well OK, fair enough, this relies on the varying level of the adult/guardian, but there shouldn't be a problem should there ordinarily!).
As a sports, technical, and military instructor trainer, I have no problem with children diving within constraints, and would encourage everyone to dive on Nitrox for the enhanced safety it provides on shallow responsible dive profiles.
As DandyDon says: Unfortunately the rules are built in for the average person and mentality, and are designed to protect certifying associations from unneccesary litigation (although some of them are still driven by reducing standards as a means of increasing profits). It's a sad truth that we all have to adhere to, which is borne from our own current propensity to sue anyone and everyone for anything and everything!
regards
Dennis