Apple Gift Card on Child Apple Account - How do we redeem?

My daughter has loaded an Apple Gift Card on her Apple account. She is now trying to do an in-app purchase and can't use the gift card to redeem.

• Her apple account is a child account.
• Her billing is linked to my credit card.
• She has to ask permission to buy.
• The Apple gift card is showing under her Apple account.

When I approve, it still uses my CC to redeem. No option to use the gift card.

Is there a way to fix this?