I added a gift card to my child's iTunes account and I cannot figure out how to use the funds. When we try to buy something it goes to Manage Payments and shows the Apple ID balance as $100 when I select it it takes me to Apple ID.

I need to know how to use an Apple Gift that was loaded to my child's account. When I try to buy something it take me to Manage Payments. Click on the card and takes me to the Apple ID Balance $100 with the option to click on it. Click on it and takes me to Apple ID Balance $100 and under that it says Redeem or Add funds. But never gives me the option to use the card!!