Looking for a book...

I am a teacher so I should know this, but I cant remember and it is driving me crazy!

A little girl in my class checked out a book at the library about a little boy who is basically a monster (or dinosaur?) in the book. The book shows the monster tearing up his room, making a mess, just being bad in general. The mom still loves him and I beleive at the end it shows him as really being a little boy.

I know it is vague, but does anyone know this book?!??! I NEED it for my son