Side note: there is only one managed user in your Plex Home. As a Plex Home admin, you can only change the Pin of this user.
The other user is a full plex.tv account. Inviting a full account into your Plex Home does not turn it into a managed user. Only the user itself can change his/her PIN.
If you created this account for one of your family members, you need to log directly into this account by using the email address and the password that you used to create this account.
(I recommend you to use your web browser’s “private”/“incognito” mode to do so.)
Ok, thanks for the clarification. Wile extremely inconvenient (i.e. my partner has no interest managing logins, password resets, etc, and the only purpose of the separate account is to not mess up playlists, progress, etc, and PINs to keep the kids in check) this makes sense.