This study aimed to investigate the associations between maternal parenting stress and children’s social competence, along with potential moderated mediation mechanisms within Chinese parent-grandparent co-parenting families. The study involved 412 mothers of children aged 3–6 years, who completed questionnaires assessing parenting stress, the quality of the parent-child relationship, the co-parenting relationship with grandparents, and their children’s social competence. The results revealed: (1) a negative correlation between maternal parenting stress and children’s social competence; (2) mediation of the link between maternal parenting stress and children’s social competence by the mother-child relationship; and (3) a moderating effect of the mother-grandparent co-parenting relationship on the relationship between maternal parenting stress and the parent-child relationship. However, no direct moderating effect of the mother-grandparent co-parenting relationship on the association between parenting stress and children’s social competence was detected. These findings highlight the significance of considering the co-parenting context in examining the correlations between maternal parenting stress and children’s social competence.