This article analyses adult and child development in the zone of proximal development in an educational practice based in Vygotsky's theories of play: the playworld educational practice. The playworld educational practice is a central component of a Scandinavian play pedagogy that promotes shared responsibility amongst adults and children for engaging in adult-child joint play. The playworld practice, which is based on a work of children's literature, includes joint adult-child scripted and improvisational acting and set design. We explore conditions under which playworld activities create a zone of proximal development that fosters development in both adult and child. Our analysis, based on data from a K-1 classroom, expands Vygotsky's concept of the zone of proximal development so that we see not only the unidirectional development of a child toward an adult stage of development but also the simultaneous development experienced by adults participating in the zone with the child.