I am also very interested in this problem. Is there a possible workaround for an adult to artificially lower his/her birth year associated with the Apple ID so he/she appears to be a child, and then create a second Apple ID that has an 'adult' birth date, and use the adult birth date to manage the 'child' account?
A little cumbersome, but I'm trying that now. Trouble is, I see that the 'child' account can easily enable / disable any app's access to the Screen Time API with an easy toggle that doesn't require the 'parent' approval.
So did I misunderstand the distinction between child-parent auth vs. individual auth?