Have someone take it away. Or hide it somewhere in the house.

You could use a wall outlet that was "switched" somehow.. remotely, etc,
but the kids would just be able to plug the TV in somewhere else.

You could completely disable the ON button (disconnect it internally)
and would need the Remote Control to turn the TV ON
(if it has a remote)

You could put a piece of clear scotch tape on one of the prongs that plugs into the wall and plug it back in. When you want to play the tv, unplug it, peel the tape off and plug it back in. Don't let the kids see you do it or they'll know what to do. When I was growing up, if we were told not to turn on a tv, we didn't (after the first time). The ringing in our ears was a reminder.

Take away the remote?

Or maybe there's an internal fuse that can be reached without disassembling the unit?