Cognitive appraisals of the trauma play a key role in children’s adjustment following a traumatic event, but how these appraisals develop is not well understood. Cognitive models of posttraumatic stress disorder (PTSD) suggest that social support is a key influence on the development of appraisals, and we aimed to explore this relationship further. As predicted by cognitive models of PTSD, we hypothesized that cognitive appraisals would mediate the relationship between social support and symptom development. Ninety seven children (Mean age = 12.08 years) were assessed at one month and six months following a single incident trauma. We administered self-report measures of cognitive appraisals, social support, and a diagnostic interview for PTSD. Results indicated that cognitive appraisals at one month post-trauma mediated the relationship between social support at one month post-trauma, and PTSD severity at follow-up. Differences in this relationship were observed between child-reported social support and parent-rated ability to provide support, suggesting that the child’s perception of support was vital in appraisal development. Firm evidence was provided for the application of cognitive models of PTSD to children.