Scare Level. I am amazed people think this movie is okay for young kids. Even during the movie, we heard little kids telling their moms they wanted to leave and wanted to go home. There were kids as young as 3 in the theater watching this movie, and they got scared. It's a little scarier than the first movie (Monsters, Inc.). Think about the scene in the first movie where Sully is captured on camera doing his scare demonstration, and the little girl sees him on the screen and starts crying. That's the level of scary in many scenes of this movie. If you're kid can handle that, great. But if your kid is the super sensitive type, this may not be a good match.
Humor. This movie is funny, just like the first one. But I would venture to say it's filled with even more adult humor than the first one. That might make it a little less enjoyable for some kids. The adult humor isn't really anything inappropriate, it's just stuff that many kids wouldn't understand.