It's scary when Endus and I agree. I need an adult. Oh wait, I am one. Nvm.
OT: Precisely this. Games have parental locks. WoW has one. Parents can decide how many hours a day their kids can play and in between what hours of the day they can. Simply add in the parental locks a toggle 'disable microtransactions' so their kids have to ask them permission before they can spend more stuff, since the credit card info is already stored on the game's website and such.