you can do this with android as well, with google play "gift cards". they are not available in many countries at this time though.
i give the kids a small amount of cash a week if they have done their part. they can then save this up and buy an itunes or google play gift card. after that, they can put that on their ipad/android store and use it as they wish. i still need to enter the password for them and look at what they are spending it and ask if they really are sure about buying those 5 items for $10 of some in app purchase for some garbage i am sure they will never look at again in a week. their choice though, and not possible to overspend.
but certainly i can see many parents in a hurry just slapping the CC there and telling the kids to sit quietly..