PS Plus accounts - I just don't get it?

PlayStation Network / PlayStation Plus accounts:
I have a PS3, PS4 and each of my 3 kids have a PS Vita.
I understand you can have 2 PS Vitas tied to one PS Plus account, but even so, one brother playing against the other on a game like Minecraft doesn't work too well, one has "Mobs" that the other can't see?
With 3 is impossible. So I've set up 3 separate ps plus accounts. (Yes, it's costing a fortune), now, when we get the free monthly games, do I seriously have to download 3 copies of the same game to the PS3, or PS4 - or am I simple?

They're called mods (not mobs). You can't play on the same PSN account on the same 2 systems (e.g. 2 Vitas or 2 PS3s) at the same time, it can only be signed into one at a time, so that's why. You can download the game once on each system. However, with PS4, it *must* the Primary account else it won't be shared and you would then have to download it separately for each User.

There's a difference between activating 2 PS Vitas at the same time with the same PSN, and signing in to two Vitas at the same time. The first is allowed, the second causes problems when it's even allowed to happen. Playing the same game with the same ID from two accounts at the same time is going to cause problems as the servers can mix up who's who since they're both "the same person".

The Vita stuff would have to be downloaded individually to each Vita. On the PS3/PS4, it should only need to be downloaded once but each individual user may need to download their own license for the game so there may be a small download for each account.