This paper reports a year-long observation of 27 typically-developing preschoolers (three-year-olds) interacting with an interactive robot, Keepon, which is a simple creature-like robot capable of expressing attention (by gazeslash posture) and simple emotions (by body movement). Keepon was placed in their playroom and tele-controlled by a human operator (wizard) in a remote room. Throughout 25 three-hour-long sessions, the children showed not only individual actions, such as approach to, exploration of, and interaction with Keepon, but also collective social actions, where the children spontaneously and actively situate Keepon in their circle of, for example, playing house. This field study suggests that Keepon's infantile appearance and capabilities would induce from the children (1) various prosocial behavior as if they took care of or protected Keepon and (2) projection of their social expectation, such as a meaning of body movement and a role in pretense play, to Keepon. The interaction data has been shared among the teachers and the parents for improving and motivating their child care practices.