I HAVE GOT YOU COVERED!
I did a report like this back in my wee old days of high school. Here are the negatives I had:
1) starts trends that can be hazardous to the health and safety of participants. For example, someone sees the "knife song game" on YouTube and says 'Hey, I wanna do that' and gets hurt.
2) Many videos are inappropriate with no way of parental control over which videos children see. For example, a four year old can see some ridiculous videos of twerking or something like that, where there is scant nudity.
3) YouTube can teach skills via tutorial videos that people can use for bad ends. For example, someone can look up how to pick locks, make weapons, etc.
Those were my cons, but my report was more Pro. that should be all you need