Background Following the Boston Marathon attack, the extraordinary interagency manhunt and shelter‐in‐place made for a truly unprecedented experience for area families. Although research on Boston youth has found robust associations between manhunt‐related experiences and post‐attack functioning, such work does little to identify the specific needs of a particularly vulnerable population—i.e., children with a relative who participated in the manhunt. Understanding the adjustment of these youth is critical for informing clinical efforts. Methods Survey of Boston‐area parents/caretakers (N = 460) reporting on their child's attack/manhunt‐related experiences, as well as psychosocial functioning in the first six post‐attack months; analyses compared youth with and without a relative in law enforcement or the armed services who participated in the manhunt. Results The proportion of youth with likely PTSD was 5.7 times higher among youth with relatives in the manhunt than among youth without. After accounting for child demographics, blast exposure, and children's own exposure to manhunt events (e.g., hearing/seeing gunfire/explosions, having officers enter/search home), having a relative in the manhunt significantly predicted child PTSD symptoms, emotional symptoms, and hyperactivity/inattention. Fear during the manhunt that a loved one could be hurt mediated relationships between having a relative in the manhunt and clinical outcomes; living within the zone of greatest manhunt activity did not moderate observed relationships. Conclusions Children with relatives called upon to participate in the unprecedented interagency manhunt following the Boston Marathon attack carried a particularly heavy mental health burden. Continued research is needed to clarify the clinical needs of youth with relatives in high‐risk occupations.