Through a document-based ethno-historical approach, this article shows how cinema in the 1920s managed to inform urban children’s games and world-building activities, contrary to contemporary assumptions from early education reformers and sociologists that informed research into children’s play. I first show how most of this research tried to prove an early version of the ‘displacement effect’ theory, constructing modern media as impoverishing children’s imaginaries by transforming them into passive spectators – a modern disease identified as ‘spectatoritis’. At the same time, this research ignored its own data that pointed to the many ways in which children were actually developing their own mode of active spectatorship, poaching material from feature films and serials to inform and organize play – a mode of spectatorship I propose to call ‘sandbox spectatorship’. I then turn to some of these testimonies from 1920s children to recover the rhythms, places and roles of this extensive re-appropriation of film texts as sandbox spaces. The article concludes by suggesting three potential avenues for more research into the history of the deployment of movie-worlds into children’s world-building play.