Again, its none. But, I just got off the phone with support. They couldn't figure out what the real cause was either, but they came up with a work around to get past it.
Here is what you need to do to fix this...
- Open Wallet on a device and add a card, it doesn't matter if you verify it or not.
- Wait minute or so, then return to your Apple ID
- Open Payment Methods <-- for me this took a very long time to update, and it seemed like it was going to fail again, but eventually it did return the new card, and allowed me to update the settings.
- With the payment process in place, it now allowed me to create the login for my child... using that card as the validation to create the account.