Co-regulation of behavior occurring within parent-child attachment relationships is thought to be the primary means through which children develop the capacity to regulate emotion, an ability that is protective across development. Existing research on parent-child co-regulation focuses predominantly on parent-infant dyads, and operationalizes co-regulation as the matching of facial expressions; however, matching can occur on other behaviors, including vocal tone, body movement, and language. Studies with young children find that greater matching is associated with children’s lower emotion reactivity, but with unknown impacts on parents. In this study we examine a recently-developed metric of behavioral matching, language style matching (LSM), a composite measure of the similarity of function word use in spoken or written language between two or more people. We test whether LSM between mothers and their school-aged children is associated with children’s and mothers’ physiological and subjective emotion reactivity. Children completed a standardized stressor task while their mothers observed; children’s and mother’s cortisol and cardiovascular reactivity were assessed, as were their subjective reports of emotion reactivity. Following the stressor, children and mothers completed independent interviews about the experience, later assessed for LSM. Higher mother-child LSM was associated with lower emotion reactivity (lower cortisol reactivity, lower reports of negative emotion) for children, and with higher maternal cardiovascular but not cortisol or subjective reactivity. Further, higher LSM was more strongly associated with lower child cortisol reactivity when mothers were more reactive themselves. We conclude that mother-child LSM, thought to reflect a history of co-regulated interaction, confers protective benefits for children, but heightened reactivity for mothers.