In this paper, we present ongoing research combining two technologies to support children's cooperative interaction: interactive playgrounds and robots. We propose that interactive playgrounds are vehicles for playful cooperation when robots are integrated into the system as cooperative co-players. We developed the Hatch 'em all game, wherein children are encouraged to hatch eggs cooperatively with a robot, and tested the effect of the robot's cooperative behavior on the children. We found that when the robot played cooperatively, children cooperated and helped the robot more than when the robot played selfishly. Our findings suggest that the social affordances of the playground, together with the social interactions between the children and the robot, enabled instances of team cooperation and prosocial behavior. Our work contributes to the CSCW community by opening a novel avenue for supporting children's cooperation, which could serve as a future test-bed to investigate the role of robots in cooperative interaction.