This is DEFINITELY NOT a kid's movies. It's an amazing psychological profile into Max. All the monsters are some aspect of Max: Judith- his low self esteem, Carol- his uncontrolled rage.
As a teacher and a mother, it's a great insight into children and their complexity. It offers an explanation for misbehavior at it's root; internal chaos and hurt.
But aside from all the psychoanalysis. The movie is simply a great flick for adults. It's very dark and the characters are well developed. As I said in the title, it's the adult's version of the book. It's everything you read into the book and everything with which you've identified in yourself when you read it.