I know many nons who are just vile and like to be controversial for the sake of it. They will say sexual thing about young girls in a very nasty and aggressive way. I do think many commenters on such videos are probably trolls or even other young kids near the girl's ages. Kids are just as capable as adults when it comes to being kind or cruel.

As for comments, I see no problem leaving complimentary comments. "You are great at stretching (yoga dancing whatever they are doing) and you have very nice legs (body, pretty face, eyes whatever)." It's a comment that acknowledges their activity and them as a person as well as their assets (no pun intended) and that is what is important. To not just view them in terms of their bodies.

It's how I deal with women I am attracted to. Acknowledge them for who they are first and secondarily acknowledge their physical features. Especially if you are being genuine. Of course, especially in our society, always assume that the person will be at least somewhat offended by more sexual compliments that focus on what society deems sexual areas of the body. Until you know them better at least. It's a safe assumption to go on because people (except males) are more than likely to be raised to be offended by such comments unless they know the person.