Although concurrent associations between parent and child posttraumatic stress symptoms (PTSS) have been well-documented, few longitudinal studies have examined bidirectional influences by modeling the effects of both parent and child PTSS simultaneously over time. The current study examines patterns of PTSS in children and their mothers beginning in preschool and continuing through elementary school age (ages 4–9 years) in a large, heterogeneous sample (N = 331 mother-child dyads). Mothers reported on their own and their child’s posttraumatic stress symptoms. A random intercept cross-lagged panel model (RI-CLPM) was used to examine associations between symptoms across six time points. Results indicated that maternal and child symptoms were associated with each other at concurrent time points and tended to fluctuate in a synchronized manner relative to their overall mean symptom levels. Longitudinal cross-lagged paths were significant from mother to child, but non-significant from child to mother, suggesting that mothers’ symptom fluctuation at one time point predicted significant fluctuation in children’s symptoms at the subsequent time point. The concurrent co-variation of maternal and child symptoms and the predictive nature of maternal symptom flare-ups have important implications for both maternal and child mental health interventions and underscore the importance of attending to mothers’ symptomatology early in treatment.