Hang on....here is the training for "using Nitrox":
1. Breathe in
2. Breathe out
3. Repeat steps 1 and 2
Training for calculating MOD, logging mix, and such is a little bit more involved but the 11yo diver's parents should be doing/overseeing that...even if the child diver was Nirtrox certified. Whether the child is Nitrox certified or not, the parent/adult diving with the child should ensure they stay well above their MOD to prevent the possibility of exceeding the MOD for the mix they are diving.
I can't fathom a reason the parent shouldn't obtain an extra Nitrox cylinder for their child if the parent will be diving a mix other than air....it actually makes more sense to do so, from a safety standpoint, particularly if the parent will have their computer set to the O2% they are diving.
-Z
