I can't think of a better way to help a child get over their "monster in the closet" phase. The movie depicts monsters as having very human-like traits--basically just like you and me except they look different. Monsters, Inc. is a great movie for teaching the value of friendship, hard work, and doing the right thing even though it hurts.
You could make Billy Crystal a serial killer in a movie and he'd still be Billy Crystal. This movie is no exception. His zany antics will leave even young kids howling in laughter. John Goodman's "papa bear" persona brings feelings of safety and security to the table as well. The friendship/camaraderie amongst them is very powerful and I think that's a good value that kids need to learn. Sully (Goodman) spend the whole movie trying to do the right thing and correct Randall's (Steve Buscemi) mistake. Randall, the bad guy, demonstrates that greed and vanity are bad, as he's constantly trying to be a more productive worker than Sully. Both monsters demonstrate a great work ethic (or in Randall's case a great desire to have a good work ethic), which is important for children to learn as well.
Having said all of that, parts of this movie will scare most young children...especially the first two minutes of the movie. Randall and his accomplice (Mr. Waternoose, voiced by the late, great James Coburn) are scary at times, but both demonstrate well the power of greed. The end of the movie offers that there are alternate and more ethical ways of making a living. It also demonstrates that doing the right thing is important, whether letting go of someone you love, or refusing to cheat in order to make it to the top.
All in all it's a phenomenal film. Disney really upped the ante on the upscaling of this one. The 1080p video of the Blu-Ray is stunning as is it's lossless HD-DTS soundtrack.