Relationship closeness determines the propensity to spontaneously reproduce another’s emotional and physiological stress response. We investigated whether psychosocial stress in mothers is causally linked to such empathic stress in children. Mothers (N = 76) completed either a standardized laboratory stressor or a stressfree control task, while their middle childhood-aged children (8–12 years old) were watching. Mother–child dyads simultaneously provided multiple cortisol, heart-rate, high-frequency heart-rate variability (HFHRV), and subjective stress samples. We found that stress-group children had a greater propensity to show physiologically significant cortisol release, especially boys. Watching stressed mothers also triggered stronger subjective, state empathy, and HF-HRV stress responses, with the latter relying on elevated trait cognitive empathy ratings. Only in the stressed dyads, children’s HF-HRV resonated with those of their mothers’. We conclude that young children, although only mildly stressed, spontaneously reproduce maternal stress.