The prevalence of smart devices in young children’s lives has changed their methods of interacting with others and their development of essential playfulness skills. However, less is known about the role of smart devices in very young children’s playfulness. To understand the relationship between young children’s engagement in smart device use and their playfulness, the role of mothers’ parenting has been considered one of the most influential factors. However, there remains a dearth of academic research on whether mothers’ actual parenting practices play a mediating role in the relationship between their parenting cognition and children’s media use. To extend the concept of parenting factors expressed through cognitions and practices, maternal rejection of children in relation to their parenting efficacy and perceptions of the impact of young children’s smart device use on the children’s development were considered. Therefore, the present study examines how maternal parenting factors are related to the relationship between young children’s engagement in smart device use and their playfulness in a comprehensive framework. The participants in the study included 278 mothers and the main classroom teachers of young children aged less than 36 months enrolled in childcare centers in South Korea. Structural equation modeling analysis was performed to assess the relationships among the variables. The results indicated that children’s engagement in smart device use through mothers’ rejection of children acted as a significant mediating effect in the association between mothers’ parenting efficacy and children’s playfulness. These findings have important implications for understanding young children’s media use and their playfulness through the context of mothers’ parenting factors and, thus, suggest a new avenue for developmental research.