This is an interesting Oscar-nominated animated short because it appears that this was a family project. The two co-directors are named 'Hubley' and so are the kids who provide the voices for the characters, so I can probably safely assume this is the case.
The cartoon consists of two sisters playing and talking much the way you'd expect young children to behave. As they talk and go on various magical adventures, the animated scenes change with them. The style of the animation is rather reminiscent of Ezra Jack Keats and Eric Carle and lots of intense colors are splashed about the screen. It looks very late 1960s and is remarkably different from the sort of animated films that were nominated back in the 1940s and 50s. This isn't a criticism--just an observation.
