Parents of children with developmental delays (DD) often report significantly heightened levels of stress when compared to families of typically developing (TD) children. While elevated levels of early parenting stress are shown to negatively impact social development in TD children, this effect may be compounded for children with DD, who are already at greater risk of experiencing social difficulties. We sought to examine whether changes in child social skills occur after parent participation in mindfulness based stress reduction (MBSR) intervention, and whether these changes were associated with parent–child relational factors. Parental stress was reduced through an 8-week MBSR training group. Changes in child social skills were measured using the social skills improvement system (SSIS), which was completed by 3 categories of respondents: parents participating in the study, a secondary informant, and the child’s teacher. Parent–child relational factors were measured using the parenting relationship questionnaire (PRQ). Data from 24 families of children with DD (ages 2.5–5) was examined in this study. Paired samples t-tests examining pre-post differences revealed that mothers, secondary informants, and teachers acknowledged improvements in child self-control. Mothers and teachers also reported improvements in empathy and engagement, while secondary informants and teachers reported improvements in child assertion. Teachers also reported improvements in children’s communication, responsibility, and cooperation. Variance in child self-control was significantly accounted for by changes in two parent–child relational factors: attachment and discipline practices. These results suggest that addressing parental mental health may enhance the efficacy of child-focused interventions by promoting parental consistency in discipline and perceived attachment (i.e. parent–child closeness).