Despite the wide usage of screen media among young children, less is known about the relationships between children's screen time and their social competence development as impacted by family-related factors. Here, we investigated parents' engagement, children's screen time, and their social competence among Taiwanese Chinese children's families at three time points as the children aged (Time 1: N = 2,037, including 991 girls, M = 3.00 years; Time 2: N = 1,785, including 868 girls, M = 4.00 years; Time 3: N = 1,749, including 842 girls, M = 5.01 years). "Research Findings": From ages 3 to 5 years, parental engagement positively predicted children's social competence, whereas children's screen time negatively predicted their social competence. Examination of the mechanism underlying these longitudinal relationships suggested that when parents engaged less frequently in interacting with children, their children tended to spend more time on screen media, and then developed a lower level of social competence during the early years. "Practice or Policy": Our findings indicate that young children's time spent on screen media should be monitored and restricted. Instead of keeping children occupied with screen media, parents and educators should engage more actively in interacting with children to promote their social competence.