Many parents have concerns about the implications of dating (and subsequent breakups) on their children’s wellbeing. Yet, little is known about the ways in which mothers’ post-divorce dating breakups influence children’s development. According to family systems theory, the effect dating breakups have on children’s behavior may be more dependent on the rapport children have with mothers’ dating partners than the event of a breakup itself. The goal of this study is to examine the effect of mothers’ post-divorce dating breakups on children’s adjustment, specifically internalizing and externalizing behaviors, while also testing children’s rapport with dating partners as a moderator. The current study used longitudinal, multi-method and multi-informant data of mothers who dated following divorce (N = 270) and children who were aware of their mothers’ dating relationships (N = 170). Using hierarchical linear modeling techniques, results indicated no main effect of breakup on children’s problem behaviors. However, children’s rapport with mothers’ dating partners significantly moderated the effects of breakups on children’s internalizing behaviors. Children who reported high levels of rapport with dating partners exhibited more internalizing behaviors at breakup compared to children who reported low levels of rapport with dating partners. Additionally, in families where children were aware of their mothers’ dating relationships and mothers experienced breakups (n = 88), rapport moderated the effects of breakup for both internalizing and externalizing behaviors. Thus, transitions out of dating relationships appear stressful for children when they established positive relationships with mothers’ dating partners. Further implications for post-divorce adjustment are discussed.