I don't know anything about the dude in the video, any controversy surrounding him, or his opinions on other things. That said, I agree with him completely here.
It's entertainment for one. Nothing that goes on in horror is acceptable in the real world. Murder, torture, dismemberment, etc are obviously morally reprehensible. That's why these films feature acts like this. They're supposed to show that the monster is just that, a monster.
And, yes, mixing teens and kids isn't the same when it comes to films. But that's only because teens in films are often portrayed by adults whereas young kids and adolescents are portrayed by actual children. In reality, they'd both just be minors. So killing a 15-16 year old high school kid would be just as reprehensible as killing a middle or elementary school kid. So, in that sense, he's correct that killing minors is pretty common in horror.
And, of course, killing children isn't new. It has been done before such as with IT. If people found that film acceptable, then they should have no problem with Buddy.
Obviously, people are allowed to feel offended about anything they want but that doesn't mean we should take their feelings seriously.