This is one of those films where it's so bad that it's almost good. It was made almost fully dependent on CG, which was one of its major downfalls. George Lopez played around 3 roles in the entire film, so it would've been nice to see a different actor. The plot didn't make much sense, but there was some symbolism. Here's the blunt meaning:
Don't give up on your dreams.
Sharper meaning:
Max's parents are going through a divorce. He's being bullied at school, and his parents and teachers aren't bothering to help. In the beginning of the film, he wishes to go to sleep all day, as dreaming is his one escape from the hell his life is. In the end, Max is clinically depressed and nobody's helping him. Instead, he's being ignored by everyone around him.
Edit: Sharkboy's choreo in the lullaby scene was pretty well done. Kudos to him.