At our house, the television is generally not on when we have company over.
If it were turned on by guests, because they absolutely had to watch a movie or tv or whatever, I reserve the right to decide if something is out of line. For example, since I have young children, and my guests want to watch "Friday the Thirteenth," it's just not going to happen. My tv, my rules. I'm the one that will be up at 3am with them while they have a nightmare.
If my kids want to watch tv, the answer is generally no, but there have been times where we've put the laptop upstairs in the playroom and all the kids have hung out in there to watch a kids' movie and chill out after play time. (This would happen, say, after they've eaten a major meal, they need some down time, and the grownups want to eat and socialise.)
If an adult went in there and turned off what the kids were watching to watch what they wanted to watch, I am not quite sure what I would do. I would think it very rude and probably let them know that this is the kids' area and invite them to rejoin the party downstairs.