Using piecewise growth curve trajectory modeling, we investigated kindergartners' physiological responses to receiving critical feedback from an adult during a laboratory drawing task. Further, we tested how children's independent self-regulated behavior, as well as the quality of parent-child co-regulation, related to physiological reactivity to and recovery from this challenge. We used respiratory sinus arrhythmia (RSA) to measure parasympathetic nervous system activity. Participants were 96 children (M[subscript age] = 5.6 years, 56% female) and their parents. We used observer ratings to capture children's self-regulated behavior during the laboratory visit and state-space grid methodology (an innovative, moment-to-moment behavioral coding method) to index observed parent-child positive co-regulation during four interaction tasks. First, the quality of dyadic parent-child co-regulation was associated significantly with children's RSA reactivity during the dyadic experience of receiving critical feedback from an adult. Specifically, children with higher levels of positive parent-child co-regulation exhibited decreases in RSA while receiving critical feedback, which may indicate active engagement or coping with the challenging situation. Second, children's self-regulated behavior was associated significantly with RSA recovery during a period immediately after the task ended, when children were seated alone. Children with lower levels of observed self-regulated behavior showed sudden RSA decrease after the critical feedback ceased, suggesting that this post-task period was physiologically challenging for them.