You can setup family accounts in iOS and anything your children want to purchase is done via request to the parent account. The parent then has to approve the purchase and the purchase is done using the parent's account details.
An argument to this I keep seeing is "well if the parent doesn't understand it then the kid shouldn't be using it." Heck, if my parents only allowed me to play with things they understood, I'd never be in a software engineering career. Parents should be able to make a mistake with a video game an not have to pay _16 thousand dollars_ to recover from it.