I like the animation style but the broblem lies in the story telling. I know you are trying to connect these imaginary creatures with this little boy, and that they will dissapear when he get's older. But i can't stop my brain from connecting all tho's creatures with the hollidays they represent (Remember it's his birthday).
when he get's a hug from both the tooth fairy and the easter bunny, he get's scared and after that scene you see the midget walk away from him. I found it a very cheap transition to the depressing part, and the midget was cut half way in the only scene he had maybe a usefull role. It's like you designed all the characters first and you couldn't really find a right way to fit them in the story.
Maybe it's the lack of time to blame. But i find it so disturbing that such a beautifull animation is put in such a bad sequence. Maybe you should put more attention in the storyboard next time.