Can my adult child add their own payment method

No. If she gives you her card to add as a payment method, it becomes the default for all charges on the account. You can disable purchase sharing, or ask the other members to add funds to their Apple ID or add Apple gift cards to their accounts; those sources are used first before your credit card.

Roseyred_911 wrote:

Please confirm. My 20 yr old adult daughter cannot add her own cards to the wallet? We have purchase share turned off.

If the Organizer turns off Purchase Sharing, each family member can use their own payment method for purchases. It is detailed in this Support Article:

How to share apps and purchases with your family - Apple Support

Correct. Only the family organizer can add a payment method. Others in the family can pay using funds on their Apple ID or Apple gift cards added to their account.

She can add her own cards to her wallet, yes. Even with purchase sharing enabled, she can add her own cards for use in stores or online, just not for shared purchases in a family share.

Please confirm. My 20 yr old adult daughter cannot add her own cards to the wallet? We have purchase share turned off.