If you are a member of family sharing, the only payment method that can be on file is the organizer's card.
See -> Make sure that each family member has a unique Apple ID
Make sure that each family member has a unique Apple ID - Apple Support
See -> How to redeem your Apple Gift Card or App Store & iTunes gift card
How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)
When redeeming a gift card, (1) the amount will be credited to your Apple ID balance and (2) then you need to set your Apple ID balance as your default payment.
(1) See -> Check your Apple ID balance
Check your Apple ID balance – Apple Support (UK)
(2) See -> Add a payment method to your Apple ID
Add a payment method to your Apple ID – Apple Support (UK)
If you update your payment method or add funds to your Apple ID balance, It would take about 1 to 3 business days in order to process.
If you're still unable to update your payment method, Contact Apple Support.
When your children purchase something from the App Store or iTunes Store, with their own Apple ID, the charges will be deducted from their Apple ID balance, before the organizer's credit card is charged.