Deleting child's account

I had issues with parental control and accidentaly created a second apple account for my child that I need to remove. I have never set up the mailbox for this account and tried to remove it from family, but it says that I can not do it until my son is 15 years old. So I just want to delete his second account that we do not need. When I type in the password for the account, it says it is wrong (but none of us ever changed it), so I try to recover it with my phone number, and after confirming with the code, the second part asks me to log in to this email adresse and provide a code, that I do NOT have access to (the reason I need a new password), in order to get into the account! So how do I reset a password and get into the account so I can simply delete it?