Android devices are not really a multi user system. Not in the sense that you or I would like it to be. With a laptop, running Mac, Windows, Linux... An administrator can lock it down and just hand it to a non-privileged user.
Phones are more personal and the various solutions that try to mimic a multi-user environment usually come up lacking in many ways. EMM or MDM solutions tend to isolate apps and data but still allow the user with physical access to have more or less complete control. There are phones built for parental controls and even dedicated hardware meant for kids.
Graphene OS was not designed for this use though. But with secondary user profiles, it's getting very close.
The main issue is that when the phone reboots the owner profile must be unlocked. So if you are okay with them coming back to you every so often to unlock the owner profile for reboots, sure it could work.