Remove Find iPhone Child iCloud account forgotten password & PIN

We forgot the iCloud account password and phone PIN for my son's phone (ok it was my ex who did it). Anyway I've erased the phone but still need to remove find my iPhone. Because his account as set up as under 13 I can't remove it from 'Find my iPhone' on my device.

I also have tried resetting the password but it asks me for the PIN used to login to his (old) phone to finalize resetting the iCloud account password (which it's also not accepting), so we get stuck there and are unable to just login to the iCloud account with a newly reset password.

The phone has been erased but since 'Find my iPhone' is not disabled it can't be resold.

How do you remove an iCloud account for someone under 13 when that device is wiped, the iCloud account password AND original phone PIN are forgotten, you don't have 2 factor authentication or security questions (I got a message to wait for Apple to authenticate me somehow??).

Is there no way to remove this forgotten iCloud account from my iTunes account which is the parent account?

Thanks!