Family Sharing Purchase non card participant

We have joined our Apple Card and have separate Apple IDs and a son that doesn’t live in the home has one card and we have the other. How is it that the daughter, who doesn’t live not live in the home, make a purchase and our card gets charged when she is not in the Apple Card account? We do share Apple Music with her. she does not have the account information or card.

When calling Apple, they said that if her card is insufficient and she down and in-app purchase, that payment would go to my husbands card as he is the organizer. How is this when she is not on the Apple Card account as a participant?