Learned something "interesting" yesterday. It was sort of an urgent situation (eager kids, Christmas Day) so I created a new thread. I posted the resolution there but figured I'd drop a note here as well as this thread obviously gets much more traffic.. So, in case it helps anyone...
It turns out that the ability for other users to play the master user's games is dependent on those games already having been downloaded such that they appear in the nav bar. If they are in the library but not yet downloaded, no joy. Why? Because if you check Library while logged in as another user, nothing will appear there. IOW the Library shows that users library, not the master user's library. I tried all combinations of activating/deactivating the PS4 as primary for the master user and other users. Nothing worked.
I finally got through to Sony support and they confirmed this behavior as correct. I suppose this makes sense as it's how the portability of one's library works so if you go to a friend's house you can play your games. But it wasn't the behavior I expected. It'll require somewhat more active management of installs and HDD space since my kids won't be able to jump into my library (unless I give them PW access) to download games.
Stuck up, half witted, scruffy looking, nerf herder.