How can I help my child remove Screen Time restrictions with forgotten password?

I’ve searched online and can’t find the answer I need. I have forgotten the screen time passcode that I set up on my kid’s phone. He’s now an adult and wants to remove it, but the instructions for changing a forgotten passcode don’t work because I no longer have his phone listed under screen time in System Settings on my device (I am the family organiser).

Are parental passcodes meant to be automatically erased when the child becomes an adult? How does he remove the passcode? He doesn’t have the ‘forgotten passcode’ option on his phone.

Thanks!

[Re-Titled by Moderator]

Original Title: Forgot Family Sharing Screen Time Passcode