The present study examines the structure and mathematical content of children's mathematical arguments as part of communication in play-based activities. It shows how Nordin and Boistrup's (The Journal of Mathematical Behavior 51:15-27, 2018) framework for identifying and reconstructing mathematical arguments, which includes Toulmin's model of argumentation, the notion of anchoring (Lithner, Educational Studies in Mathematics 67:255-276, 2008) and a multimodal approach, can be used to identify and explore preschool children's mathematical arguments. Two different types of argument that occurred during play-based activities were identified: partial arguments and full arguments. The findings reveal the extensive use of multimodal interactions in all parts of the children's mathematical arguments. Moreover, the findings point to the crucial role of adults as dialogue collaborators in the argumentation that emerges in the play-based activities.