My screen time passcode

The Apple ID would usually be the same one used for a device transfer and preserved by the device transfer, but it seems there’s also a new Apple ID created here?

The Apple ID accounts should all still be in Family Sharing with a parent/guardian role, so maybe the missing Apple ID can be identified located through that.

If you forgot your Apple ID - Apple Support

If you forgot your Apple ID password - Apple Support

Shows how to see members: Leave Family Sharing - Apple Support

You’re probably headed for a discussion with Apple directly:

• To reset your password, you need to know the email address for your Apple ID. Don't know the email address for your Apple ID?
• If you still can't sign in with your Apple ID and password, make sure that you entered the correct Apple ID. In most cases, your Apple ID is also the primary email address of your Apple ID account.
• In some cases, you might be able to speed up the account recovery process or reset your password immediately by verifying a six-digit code sent to your primary email address.
• Still need help? Contact Apple Support. 

Log some feedback with Apple here too, as displaying the holder of the parent/guardian role including their Apple ID on the managed device would be useful here.

I’d also suggest reversing this whole screen time management configuration, and managing the parents’ devices and and not yours, as the parents here do seem to be having some difficulty with basic operations, with device transfers, and maintaining security.