Show them X free levels and with free hints.

When they get into the groove, at X+1 level show them "Did you like this? You can get 200+ levels if you convince your parents that this is a worthwhile investment for your learning." (copy TBD) and bam, you have a traditional game with a demo and a way to buy it that doesn't train kids to expect in-app purchases for every breath they take.

And btw, $25 is high even for an indie steam game, a mobile game will be even harder to market at that price. Just FYI. Best of luck!