I mean, c'mon. It's YouTube.
There are some mature themes that are relatively common, but you're really not going to find anything absolutely horrendous on here. If you're a parent, just keep an eye on what your kids are doing, and you will most likely be fine.
If you ARE a parent, and you are letting your children on this site, the most you probably have to worry about is profane language. There's definitely a lot of that on here.
This site is the PERFECT place for people of ALL ages! Even though COPPA screwed up the service, I still like it because the Content Made For Kids feature helps me figure out quickly if the video I want to watch is also safe for my daughters.