The current study provides the first systematic review and meta-analysis of the associations of smartphone and tablet use with psychosocial, cognitive, and sleep-related factors in early childhood development. The meta-analysis aimed to provide an overall assessment of the evidence while the systematic review offered a rich overview of the methodological approaches adopted to assess these associations. Studies were included in the review if they examined the association of smartphone or tablet use with a measure of psychosocial development, cognitive development, or sleep in toddlers or preschoolers. Out of 1050 articles that were initially identified, 26 studies were included in the final sample of the systematic review, of which 19 were included in the meta-analysis. Data were screened, extracted, and synthesized according to PRISMA guidelines. A random-effects meta-analysis of correlations found a significant yet weak association of increased smartphone and tablet use with poorer overall child developmental factors.Additionally, a similar but stronger association was found between parental perceptions of problematic device use and poorer overall child factors. Meta-correlations with device use were significant for sleep, but not for psychosocial and cognitive factors. Overall, the results suggest that longitudinal cohort and experimental investigations would elucidate more causal relationships of child factors with smartphone and tablet use. Employing multiple methods of screen-use assessment, and considering the multiple levels of proximal and distal influences on child smartphone and tablet use, would also be useful. Adopting more rigorous research practices in the future, will facilitate deeper insights into the potential developmental implications of smartphone and tablet use in early childhood.