[dubya]

Not mobile as far as I know, but Prodigy (prodigygame.com) is allowed/encouraged at my kid's school and has a lot of these traits. There are treasure chests at the end of a battle, gems or coins collected, and a constant reminder that subscribers get more rewards. Many of his classmates subscribe.

[cletusw]

Ugh, yes. My kids play this one at school and when I read about the game mechanics I was super disappointed. Luckily they stopped begging me to subscribe eventually since I wouldn't budge.