Switching to a different user within a Plex Home definitely doesn’t require a completely new login.
If they have created full plex.tv user accounts for each family member, they can invite them into the same Plex Home, thus enabling Fast User Switching.
Inviting full plex accounts into a Plex Home is a Plex Pass feature.
As for the first part of your request: there is already an existing feature request for it. Allow managed users to have access to shared content
