I need to erase my credit card number from my grandsons account
I need to remove my credit card number from the account
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Unless you can log into his account (preferably using a web browser) you will have to ask him to do that. Either that or else ask your bank to issue you with a new card.
How did this happen in the first place? Are you a Family Sharing Organizer? If yes, you may be confusing how that works. From: How to share purchases with your family - How to share purchases with your family - Apple Support
"When you turn on purchase sharing, everyone in your family gets access to apps, music, movies, TV shows, and books that family members buy. The family organizer is billed for family members' purchases.