In this study we investigate how 5-year-old children in Brazil and their teachers collectively design science curriculum. More specifically, we develop an agency|structure dialectic as a framework to describe this collective praxis in which science curriculum may emerge as the result of children-teacher transactions rather than as a result of being predetermined and controlled by the latter. We draw on a cultural-historical approach and on the theory of structure and agency to analyze the events showing the complexity of the activity inside a classroom of very young children by science education standards. Data were collected in the context of a science unit in an early-childhood education program in Belo Horizonte. Our study suggests that (a) throughout the movement of agency|passivity || schema|resources one can observe participative thinking, a form of collective consciousness that arises in and from lived experience; (b) learning is a process in which a group is invested in searching for solutions while they create schemas and rearrange resources to evolve a new structure; and (c) the emergent curriculum is a powerful form of praxis that develops children's participation from early childhood on.