If you've managed to track down discrete on/off commands for your TV and receiver you shouldn't have a problem.

If the kids have already turned the TV on and you want to:

1) Start watching the TV. Simply press the "Watch TV" activity - it should send the discrete "on" command - which won't do anything as the TV is already on. You can now control the TV as you want it.

2) Turn the TV off. Assuming the Harmony is in the 'off' state, press the "Help" button, this should send the discrete "off" command to the TV to switch it off.