I believe that it is an early 80's movie. It starts as a movie and has a little girl that comes across a doorway and as walking through it she turns into a cartoon. The cartoon style is that of the illastrator of the dr. suess the grinch that stole christmas. She goes on some adventure... ends up back in real life in the end of the movie.

Sorry if this is vague. This is what I can remember. Any help is much appreciated.

It's a boy, but possibly The Phantom Tollbooth (1970) (http://www.imdb.com/title/tt0064806/) ?