There was a story a while ago about a group of friends that wander into a carnival fun house. In particular one of them takes on babyish traits: smooth pale skin, a shrunken member, and has trouble talking. As the story ends they flee early and the narrator notes the changes wouldn’t have been permanent if they had just rolled with it and stayed the night. It was an awesome story and I’d love to revisit it!
I think you’re probably looking for “Something Wicked” by The Screaming Moist.