Just in case you want to keep your card details, and most don't, but you can turn off in app purchases.
General > Restrictions > Enable Restrictions > Turn off in app purchases.
Otherwise, click to the right of your name in iTunes, then Accounts, then Edit Payment Preferences, and click 'None'
I do this, and simply add iTunes vouchers as I need them. With kids, I would also add the in app restrictions as well though.