Fantasy movie, most likely 80's

I think it started, or possibly ended, with a child on a long tree swing. Two siblings, boy and girl, go on an adventure, and I think it was to find their mother? She was a dancer or ballerina maybe. I don't remember most of the movie, but pretty much they ended up in some fantasy world where the people who lived there went down this big slide and were chosen to either be some sort of fairy, or a lumpy pile of earth or mush or something that sprayed glitter out of its mouth, and people actually wanted to become one of the lump creatures, I think they were revered because they had greater magic or something? The sister became a fairy and the brother became the lumpy thing. I think they went back to the real world at the end.