Children's oral language skills in preschool can predict their success in reading, writing, and academics in later schooling. Helping children improve their language skills early on could lead to more children succeeding later. As such, we examined the potential of a sociable robotic learning/teaching companion to support children's early language development. In a microgenetic study, 17 children played a storytelling game with the robot eight times over a two-month period. We evaluated whether a robot that "leveled" its stories to match the child's current abilities would lead to greater learning and language improvements than a robot that was not matched. All children learned new words, created stories, and enjoyed playing. Children who played with a matched robot used more words, and more diverse words, in their stories than unmatched children. Understanding the interplay between the robot's and the children's language will inform future work on robot companions that support children's education through play.