In order to change your payment method to, or from a credit card, simply open up iTunes.
Select your account in the top-left corner and choose “Account”
This will show you all your details relating to the account in question, including the payment method. Choose “edit” next to your payment method.
From here you will be able to change the method completely, maybe switch from CC to None, or vice versa.
Either way, be sure you know which method you have chosen last, and change it when necessary.
Of course, there are a couple of other things you should keep in mind when letting the little monsters use your iPad/iPhone for “educational” games.
Restrict in-app purchases.
On the device, go to Settings, Restrictions, Enable Restrictions, choose a passcode, and enable 2 settings:
- In-App purchases
- Require password – Immediately or in 15 minutes (remember the password for 15 minutes)
Thanks to Krypted for the insight on the credit card story.
Hope this helps some.
Enjoy!