In most cultures, play seems to matter a great deal to young children. This is evidenced by the vast amount of time children spent playing and the combination of often unsurpassed passion, imagination, and energy which they invest in this activity. This paper explores why play matters through the lens of Bakhtin's dialogic approach combined with Vygotsky's developmental theory. In expanding upon their insights into a framework termed "the transformative activist stance," we suggest that play offers unique opportunities for children to develop and exercise their agency, identity, and voice. While playing, according to this perspective, children discover how to be agentive actors--that is, unique persons who have an irreplaceable role in co-authoring social interactions, communal practices, and the world itself. In this complex endeavor, children sort out the difficult challenge of becoming unique, self-determined, and free persons within the communal world shared and co-created with others. Examples from video recordings of children's interactive play in a naturalistic setting illustrate how play paves the way for children to collaboratively create and transform the world from their unique stances and positions. This approach suggests that play is serious work for children as they develop capacities for agency. Therefore, it is critically important that early education allocates ample time for this activity.