Please help. I have racked my brain trying to figure this out. But its an old movie with a monster under the bed. He eats peanut butter and onion sandwhiches. There's a whole world under the bed and the monster who is their friend takes them down there and they almost get stuck down there.

I think it's

http://www.imdb.com/title/tt0097758/

Little Monsters (1989) Howie Mendel and Fred savage

That's it! God that seems so simple now. LOL! Thank you so much. Now I can sleep tonight.