Parent–child agreement on measures of child posttraumatic stress disorder (PTSD) is moderate at best, and understanding of this discrepancy is limited. To address this, we conducted an item-level investigation of parent–child symptom agreement to examine the potential influence of parental posttraumatic stress symptoms (PTSS) on parents’ reports of their child's PTSS. We also examined heart rate (HR) indices as possible independent indicators of child PTSD, examining patterns of association with parent versus child report. Parent–child dyads (N = 132, child age: 6–13 years, 91.7% White) were recruited after the child's hospital admission following an acute, single-incident traumatic event. At 1-month posttrauma, questionnaires assessing children's PTSS (self- and parental reports) and parental PTSS were administered. For a subset of participants (n = 70), children's HR recordings were obtained during a trauma narrative task and analyzed. Parent and child reports of child PTSS were weakly positively correlated, r = .25. Parental PTSS were found to be stronger positive predictors of parental reports of child PTSS than the children's own symptom reports, β = 0.60 vs. β = 0.14, and were associated with higher parent-reported child PTSS relative to child reports. Finally, children's self-reported PTSS were associated with HR indices, whereas parent reports were not, βs = −.33–.30 vs. βs = −.15–.01. Taken together, children's self-reported PTSS could be a more accurate reflection of their posttrauma physiological distress than parent reports. The potential influence of parental PTSS on their perceptions of their child's symptoms warrants further consideration.