Hello Jeanette293,
It sounds like you would like to remove your credit card info from your iTunes Store account. This section of the following article should help you do that:
iTunes Store: Changing your payment information
http://support.apple.com/kb/ht1918
Change your payment information on a Mac or PC
If they have their own device, you can also restrict in app purchases with Restrictions to prevent this happening again:
iOS: Understanding Restrictions (parental controls)
http://support.apple.com/kb/ht4213
Thank you for using Apple Support Communities.
Cheers,
Sterling