In J M Barrie's play, 'Peter Pan', a small house was built around the central character, Wendy. (That's where the name Wendy was invented, too, by the way.) As a result, since the 1940s, any such little house for children to play in has been known as a 'Wendy house'. It was originally just two sheets of plywood hinged together, which could be opened up, tent-like, to make an enclosed play-place.