Having said that. I just cant help having the feeling that 99% of what you are doing with your kids is lost on them. I.e. they are not there yet where they can actually appreciate and use all the information they are receiving.
I went through the same things with my son and music. As a former musician myself I was of course eager to have him learn to play an instrument. But more or less any music teacher i talked too basically said that before 3 learning an instrument was lost on them.
So instead we sing songs and I have bought a bunch of proper instruments that he can play around with as he wants.
On the iPad have have a very interesting synt app called MorhWiz and where he like to fiddle around with the other instruments he loves using MorphWiz.
This I believe is because the MorphWiz takes away some of the difficulties while still allowing for exploration. And there he actually plays stuff.
My point is. For your kids, the Ipad is like a linux system they can do everything they can think of with. They are not yet ready and cannot yet internalize the way you seem to think they internalize.
Again it's not my children, I am not in disagreement with your goals. But I think your kids could get even more out of this if you started from where they are not where you are.