Background Sexual assault is associated with a high risk of developing PTSD. Little is known about the PTSD onset in children who have recently been victimized by sexual assault. It is important to identify children at risk for PTSD after sexual assault to prevent chronic problems and revictimization. Objective The first aim of this study was to describe the development of post-traumatic stress symptoms in the four weeks after sexual assault. The second aim was to analyse whether pre-assault factors, assault-related factors, social support, and post-traumatic stress, measured at two weeks post-assault, were associated with an indication of PTSD. Method From January 2019 to March 2021, data were collected of victims aged 8–17 years ( n = 51; mean age = 15.00; SD = 1.78) who had contacted a Sexual Assault Centre. Severity of post-traumatic stress symptoms was measured at two and four weeks post-assault. The study was designed to use a multivariate logistic regression analysis. The study included female victims only. Results Most of the victims (58.8%) showed a decline in the severity of post-traumatic stress symptoms in the four weeks after sexual assault. However, 27.4% showed an increase and 13.7% showed no change in symptoms. More than two-thirds of the children (70.6%) showed severe post-traumatic stress symptoms at four weeks post-assault, i.e. had an indication of PTSD. Since only one significant difference was found, the multivariate analysis was not executed. A significant difference was found between severity of symptoms at two weeks and an indication of PTSD at four weeks ( t (49) = −5.79; p < .001). Conclusion Children with high levels of post-traumatic stress at two weeks post-assault are at risk for PTSD indication at four weeks post-assault. Further research is needed to determine whether early trauma-based treatment for children with high post-traumatic stress symptoms can prevent the development of PTSD.