One of my children has two Apple IDs. I want to delete one account, it’s shouldn’t have been setup. I need to remove this account from family sharing as I want to add my wife (there are six in our family)

I need to delete a child account, they mistakenly have two. Keep one, delete the other.

The reason this is important is that I want to use the last remaining family member position for my wife - we have four children.

All research I’ve found says:

1. you cannot remove a child from family sharing until they’re 13
2. you can transfer a child to another family

Neither of these solutions is viable, so please let me know if there is a solution.