I have a very vague memory of this movie – I think it was a short and I think it was possibly with puppets similar to Dark Crystal.  Kids are on a raft going down a river. Its dark and stormy and they see a building and they go to check it out. They look into the building and its full of evil monsters. the next day they continue down the river and they arrive at a spot that has lots of good monsters. they tell them about the bad monsters and then I think there was a monster war.  This has been bugging me for years!!