This study explores the developmental change of mother-child interactions in order to investigate which aspects of maternal behavior affect children's speech production. To this end, the interactions between 79 German-speaking mothers and their two- or five-year-old children were observed at two time points (12 months apart) and in two interactive settings (joint picture book reading and joint play with toy blocks). Measures were obtained for the speech (mean length of utterance) of mothers and children and the pointing behavior and prosody of mothers. The results suggest that mothers adapt their behavior to the advancing abilities of their children. Moreover, mothers' speech input was a significant predictor of children's speech production later in life. Interestingly, speech production of younger children was explained by the relatively simpler speech of mothers during joint play, whereas speech production of older children was explained by the relatively more complex speech of mothers during joint picture book reading. Mothers' nonverbal behavior, in contrast, did not predict children's speech production. Taken together, this study provides compelling evidence for the changing role of mothers' interactive behavior in supporting children's language production during development.