All you need is a smartthings – compatible plug which has energy monitoring. There are many of these in different protocols.
Once you get one that has energy monitoring, there are many different ways to set up rules to either log the usage or turn the device off when a power allowance has been exceeded.
Depending on the age of your kids, you will probably also need a plastic lock box to go over the outlet or they can just unplug the TV from the smart plug and plug it back into the regular socket.
So it depends on whether you want to:
A: just log the usage to review it later or
B: log the usage and receive a notification on your phone if a specific value is exceeded or
C: have the plug turn itself off after a certain amount of time
That would then determine what automation you would use.