A film about two children who get a package that turns into a portal, they get transported and the children become cartoon. The children have to get from place A to B with some help of one of the residents. I remember that during one part of the trip they are traveling in a red car, in a forest who makes them sleepy, but if they go to sleep the small "monsters" (who are cute) will kill them / eat them up.
I've got this idea that it is a Dr. Seuss movie, but still can't find it. It is of that kind of film. Would guess that it is from the 70s. (sorry for bad english)

The Phantom Tollbooth (http://en.wikipedia.org/wiki/The_Phantom_Tollbooth_%28film%29)