You make a rule. This isn't a technology problem.

My kids are the kind with the attitude that "rules are made to be broken", which I like. It's like the hacker's ethos almost.

Why bother having a rule if you're not going to enforce it? I agree that questioning authority is healthy, but people also are required to follow rules on occasion and it's your job to teach them how to do that. Using a technological solution to a discipline issue is a short term bandaid for a long time problem.

"Don't turn on the TV or <punishment>."

Even if our kids turned on the TV, there would be nothing to watch on it except the Chromecast homescreen. No cable, no antenna. Elder child's phone has no video apps on it except the built-in one they can play their own home videos on (maybe they could cast those to the TV?).

yeah, that's my definition of "TV" nowadays, it's just external display for content I push there, though I have smart TV with apps, so there is sort of way to display something, but not the traditional passive TV channels