Minecoin purchase issue

Folks:

My son received an apple gift card. It's loaded on his account - he has a family account.

He is trying to purchase Minecoins in Minecraft. I approve the transaction. When he clicks buy, it shows the account balance - but asks to validate the CCV of the credit card associated with the account. (It's an Apple screen outside of minecraft). This is unusual, as credit card validation is not needed apple gift purchases. Even more odd, the window is locked, and will not allow entry of the CCV code.

I have even tried temporarily turning off purchase approvals, and the same issue occurs.

Anyone help?