I read this book when I was young in the 80s. It was about a brother and sister (I think) who were caught in a rainstorm and happened upon a sagging, scary looking tent. They went inside and magically it was a bright, exciting room with some kind of delicious food (ice cream? cotton candy?) and I think a pinball machine? I feel like there was a witch involved, who turned out to be a good witch, but maybe I’m making that up.