The purpose of the current study was to examine the risk factors of young children's smartphone addiction in a longitudinal study design. Data collected from 313 participants (mean age, 4.5 ± 0.82 years; male, 49.8%) over 4 years for Kids Cohort for Understanding of Internet Addiction Risk Factors in Early Childhood were analyzed in this study. Mixed effect models were used to evaluate the influence of various variables on the repeated measures of smartphone addiction tendency in young children over time. The multi-level analysis showed that parents' lack of control over children's smartphone use ( = -4.523; 95% confidence interval [CI], -7.32, -1.72), and parents' higher smartphone addiction proneness ( = 6.340; 95% CI, 0.23, 0.440) predicted higher smartphone addiction tendency in young children. The responsibility of the parents to prevent their children from becoming addicted to smartphones should start in a very early age when they start using the smartphones.