Then why would you give your kids access to your credit card for the app store. Doing so is like dropping them off at a toys store with your credit card.
There is a simple solution. Have them make an account and if they want a game give them a gift card. If they get the gift card, there will be no link to your credit card. This way they can buy games on their iPhone and not have it hooked up to your credit card.
Just educate yourself on how technology works. Save yourself the trouble of a 3000 dollar IAP bill.