Objective This study aimed to examine the severity of posttraumatic stress disorder (PTSD) symptoms in children of parents with cancer and to identify individual and family factors associated with these symptoms. Methods The sample consisted of 175 children (52% girls, aged M ¼ 11.98, SD ¼ 3.20, range ¼ 6–20 years) from 92 families, of which 90 parents with a current or past cancer diagnosis and 71 healthy co-parents also completed questionnaires. Children reported on PTSD symptoms, trauma-related cognitions, emotion regulation difficulties, general family functioning, and family communication. Both parents reported on their own PTSD symptoms. Associations were investigated using multilevel regression. Results Twenty-seven percentage of the children showed clinically relevant PTSD symptoms. Intraclass correlations indicated that children from the same family showed little overlap in these symptoms. Multilevel analyses showed that child trauma-related cognitions and emotion regulation difficulties were related to higher levels of PTSD symptoms at the individual level. General family functioning was only related to child PTSD symptoms at the family level. Child PTSD severity was unrelated to parental PTSD symptoms and family communication at the family level when taking into account the other factors. Conclusions The current study highlights the psychological impact of parental cancer on children. Individual factors contributed more strongly to child PTSD symptoms than family factors. Trauma-related cognitions and emotion regulation difficulties might be targeted through specific psychoeducation for children and parents, family-oriented support and interventions, and evidence-based treatments for child PTSD