I'll try an utilitarian argument: because my younger daughter (13) has nightmares after seeing a violent video. (Not blaming YT for that, just saying that you can make a non-moral argument against violent videos in kids' channels.)
That argument will easily prove that all videos are inappropriate for Youtube Kids. An actual utilitarian argument would balance your daughter against the kids who liked seeing violent videos, and she would lose.
Furthermore, utilitarianism is a moral position. Why would you prefer a world where your daughter doesn't have nightmares over one where she does?
Your daughter would probably stop having nightmares if you gave her more exposure to that kind of video, too.
An alternative, advertisers who target children don't want to be included on videos that feature graphic depictions of violence so violent videos (or overly sexual, vulgar, etc) on YouTube Kids will be a net negative for YouTube.
