Keepon is a small creature-like robot designed for simple, natural, nonverbal interaction with children. The minimal design of Keepon's appearance and behavior is meant to intuitively and comfortably convey the robot's expressions of attention and emotion. For the past few years, we have been observing interactions between Keepon and children at various levels of physical, mental, and social development. With typically developing children, we have observed varying styles of play that suggest a progression in ontological understanding of the robot. With children suffering from developmental disorders such as autism, we have observed interactive behaviors that suggest Keepon's design is effective in eliciting a motivation to share mental states. Finally, in developing technology for interpersonal coordination and interactional synchrony, we have observed an important role of rhythm in establishing engagement between people and robots. This paper presents a comprehensive survey of work done with Keepon to date.