As been stated before, you are not able to use the same account on multiple systems simultaniously. As soon as you log in on another system, the other will log out, because each account has an ID and that ID can only be online once. Else you can't who actually is using the account.
If you want them all to play together you have 2 options.
Pay for multiple PS-plus accounts as well as the games. (This is the only way they can all play online)
Find a game which supports multiple players on 1 console. I can't tell you any new games which support this (split-screen), but on PS2 there was Need for Speed and FIFA if I remember correctly.
For your information, pretty much any system (PC, XBox, PS) only allow every account to be logged in once. Don't get your hopes up and try to get around this. Unless you violate the ToS there is no way you can achieve what you want to.