This paper reports our longitudinal observation of unconstrained child-robot interaction at a daycare center for autistic children. We used a simple robot, Keepon, that is only capable of expressing its attention (directing its gaze) and emotions (pleasure and excitement). While controlled by a remote experimenter, Keepon interacted with the children with its simple appearance and actions. With a sense of curiosity and security, the children spontaneously approached Keepon and engaged in dyadic interaction with it, which then extended to triadic interactions where they exchanged with adult caregivers pleasure and surprise found in Keepon. The three-year-long observation suggests that autistic children possess the motivation to share mental states with others, which is contrary to the commonly held position that the motivation is impaired in autism. Autistic children, however, have difficulty rather in sifting out socially meaningful information (e.g., attention and emotions) from flooding perceptual information. Keepon, with its minimal expressiveness, directly conveyed socially meaningful information to the children, which activated their intact motivation to share interests and feelings with others. We conclude that social filtering is one of the prerequisites for interpersonal communication and that robots like Keepon can facilitate social filtering and its development in autistic children.