Children's emotion regulation (ER) is associated with positive cognitive and psychological outcomes, yet little is known about whether and which aspects of early experiences foster its development. This study examined the longitudinal association between caregivers' elaboration during jointly constructed narratives with their children about past and future events and children's ER in socially challenging situations, as well as the role of theory of mind (ToM) in this association. Thirty-six Chinese caregivers and their preschool-aged children (23 girls) participated at two time points, 1 year apart (Mage at T1 = 53.63 months). At T1 and T2, caregivers engaged in a narrative task discussing both past and future events with their child. Caregivers' utterances were classified into two elaboration categories: elaborative questions, which elicited children's responses to contribute new information to an event, and elaborative statements, which provided new information about an event to children. At T2, children's ER was assessed using the Challenging Situations Task and their ToM was also measured. Findings showed that caregivers' elaborative statements during narratives at T1 predicted children's competent responses to challenging social situations 1 year later at T2, indicating stronger ER. At T2, caregivers' elaboration was not concurrently associated with children's ER. Children's ToM was significantly related to their ER, and preliminary evidence suggests that ToM might mediate the positive association between caregivers' elaborative statements and children's ER. Findings highlight the important role that caregiver-child interactions play in supporting socioemotional development and their implications for understanding the underlying mechanisms.