RecruitingBehavioural interventionMultiphase Optimization Strategy (MOST) for Treating Pediatric Post-traumatic Stress Disorder (PTSD)The goal of this clinical trial is to test the extent to which different treatment components work to improve pediatric post-traumatic stress disorder (PTSD). It will also provide evidence for how these components work. The main research questions are: What are the effects of different components used to treat PTSD? What do these components change to produce benefits in PTSD? Researchers will: Compare components to a psychological placebo to estimate their effects and measure how they work Examine how components work alone and in conjunction with other components Participants will: Receive different combinations of components and placebo Attend weekly treatment sessions Provide information to evaluate changes in PTSD