Polyvagal theory provides a framework for understanding connections between children's autonomic regulation, cognitive functioning, and behavioral adjustment. Parasympathetic regulation has been associated with executive functions and externalizing problems (EP), and children with EP demonstrate deficits in inhibition of prepotent responding, or inhibitory control (IC). We examined parasympathetic regulation of cardiac reactivity during two IC tasks in 144 children (M = 5.61 years, SD = 1.09) ranging from low to clinical levels of EP. Overall children with more EP evidenced greater RSA suppression during IC tasks than did children with fewer EP, and degree of RSA suppression also moderated associations between IC performance and EP. Only for children who showed stronger RSA suppression was accuracy of IC response inversely associated with EP, and latency of response for one task positively associated with EP. This study provides insight into the role of parasympathetic mechanisms in children's cognitive regulation of impulsive and aggressive behaviors.