Objective: To examine as secondary analyses the effect the FAMily-Oriented Support (FAMOS) family therapy program on reducing parent-reported medical traumatic stress in the sub-sample of pediatric cancer survivors, age 2-5 years. Methods: The FAMOS study was a national multicenter randomized controlled trial with all four pediatric oncology departments in Denmark (Clinicaltrials.gov [NCT02200731]). Families were randomized in parallel design (1:1) to intervention or usual care. The FAMOS program includes seven home-based psychotherapeutic sessions and is based on family systems therapy to address the individuals in the family system using cognitive behavioral, problem-solving and goal-setting techniques. Questionnaires were completed by parents at baseline, 6, and 12 months. In linear mixed-effects models, the effect of FAMOS on reducing children's trauma-related behavior after 6 and 12 months was examined in 62 children (31 in the intervention and 29 in the control group, respectively). It was also examined if a trauma-related behavior effect was mediated through reduced symptoms of depression in mothers and fathers, respectively. Results: On average, children in the intervention group experienced significantly larger decreases in trauma-related behaviors at 6 and 12 months than the control group (predicted mean difference -3.89, p =. 02 and -6.24, p =. 003, respectively). The effect on trauma-related behavior was partly mediated through reduced symptoms of depression in mothers, but not fathers. Conclusions: Adding to previously reported positive effects of the FAMOS intervention on parents' symptoms of post-traumatic stress and depression, significant improvements were found in young children's trauma related-behavior. Further research is needed to develop therapy for children with cancer.