Here is my quick review of Where the Wild Things Are.
First and most importantly, it is NOT a kids movie. The story and themes surround emotions and not simple ones. Lots of kids were in the theatre around me and I heard a lot of questions being asked through out the movie. Additionally it's very REAL. A monster or scary part in an illustrated book is one thing, but there are moments when it gets very real and very scary. In the book a threat to be eaten by one of the monsters is easily dismissed. Not so much in this movie. Additionally the movie paints the place as a real location, not something in his imagination.
For adults it has depth. Max is identifiable for many because many of us went through similar things as a kid. That's where this movie shines. There is an innocence to it but also realness. That realness that would confuse or scare children is also there for adults, but adults can understand it and relate it.
Buy your young children the book and go to the movie yourself first. If you feel they are up for it, take them then.
I know I've already beaten this drum a bit, but the 'realness' also applies to the effects. The monsters and their home look and feel very real. I was taken out a couple times. A dog and some owls didn't look as polished as the rest, but for the most part it was very well done. Good for Spike Jones using monster suits instead of CG.