This study examined the relationship between acute cortisol responses to trauma and subsequent PTSD symptoms (PTSS) in children and their biological mothers. Urinary cortisol levels were assessed in 54 children aged 8–18 upon admission to a level-1 trauma center. Six weeks posttrauma, 15-hour urine samples were collected from children and their mothers. Depression and PTSS were assessed at 6 weeks (N = 44) and 7 months (N = 38) posttrauma. Higher child in-hospital cortisol significantly predicted 6-week child PTSS. This was true only for boys at 7 months. In mothers, lower 6-week cortisol levels significantly predicted 7-month PTSS. Results extend findings of differing directions of acute hormonal predictors of PTSS in adults versus children to a sample of genetically related individuals.