Executive attention, an underlying mechanisms enabling self-regulation, can be behaviorally indicated by post-error slowing (PES) – a delay in reaction time following an error. PES develops during early childhood – plausibly shaped by genetic and environmental factors. We tested whether mothers’ and children’s PES predicted their post-traumatic stress disorder (PTSD) symptoms during a real-life stressful situation, and how each one’s PTSD symptoms moderated the other’s. Ninety-five kindergarten-aged children and mother pairs participated. In T1, participants’ PES was measured during a laboratory task. About 1.5 years afterwards, six months after a national traumatic event, mothers reported their own and their child’s PTSD symptoms (T2). Key findings show that for mothers with high PTSD symptoms, children with more developed PES at T1 showed less PTSD symptoms at T2. In contrast, for mothers with low PTSD symptoms, children’s PES was unrelated to their PTSD symptoms. For mothers of children with high PTSD symptoms, those with less developed PES at T1 showed high PTSD symptoms at T2. Mothers of children with low symptoms showed no such relation. The models explained 61.1% of children’s and 51% of mothers’ PTSD symptoms. These findings provide evidence for the protective effect of self-regulation against PTSD, and the mutual dyadic moderating effects of its manifestation.