Families of children with neurodevelopmental disorders have developmental, behavioral, and social-emotional needs that affect quality of life (QoL). This study assesses the validity and utility of a caregiver QoL measure; characterizes QoL in families with children with Down syndrome (DS), autism spectrum disorder (ASD), and a dual diagnosis of DS and ASD (DS + ASD); and compares and explores differences in QoL based on diagnosis. Caregivers of children and adolescents with ASD (n = 610) and DS (n = 177) completed the Pediatric Quality of Life Inventory Family Impact Module 2.0, yielding overall, parent functioning, family functioning, and subscale scores, and a Parent Global Impression (PGI) rating. An ASD cohort (n = 177) was sex matched to the DS cohort (n = 177) to mitigate potential sex bias. Additional analyses compared these groups with children and adolescents with DS + ASD (n = 37). Analyses showed that the Pediatric Quality of Life Inventory was valid and reliable in DS, ASD, and DS + ASD populations. No differences were reported in PGI ratings among groups. Caregivers in the DS group demonstrated higher QoL and family functioning compared with the ASD and DS + ASD groups. The DS group reported significantly better Emotional Functioning and Communication and less Worry than the ASD group. Compared with the ASD group, caregivers of the DS + ASD group indicated more concerns with Physical Functioning. Notably, the DS + ASD group had significantly lower levels of QoL than the DS group in nearly all caregiver functioning domains. This study highlights differences in QoL within and between neurodevelopmental disorder groups, which may help identify families requiring additional support, advocacy, and community engagement.