A parent can reassign a card if needed. Re-assigning a new credit card to a child means that child will be using this credit card for future payment. However the original card remains as default, for future new children of the parent, it will be using the default card, unless parents want to change it.
To reassign a card:
On the side bar, expand it by clicking
Click on Payment Details
Click Add a Card
Parent to choose preferred method of payment and update the details
Parents can make this new credit card as default or just keep as is.