I do not think my niece is wrong at all, like you said she is a child. BUT where she goes is not my responsibility and I am not responsible for looking after a child, especially after I told 3 grown adults that I am busy doing important work for my course. It is also my TV in my private space so I can put on whatever I like so if she comes into my room (after 3 adults knowing she shouldn't go in there) that isn't my problem. My brother is her father and should have been monitoring her, if not him it would be our parents.
Bit of advice, don't tell commenters on AITA not to down vote if they disagree. You may have your way of goving out votes (no problem with that at all) but this sub is for people to cast a vote on who they agree with. The logic you have falls more in line with how r/unpopularopinions works. It's like if people downvote me that is entirely fair as it is their chose, they can agree or disagree but from what I have seen on this subreddit asking someone not to downvote you will get you downvoted.
I'm a woman, not a guy. Thought I would clear that up.