Ok, this is maybe the most clickbait-like title I ever wrote, but the answer is: not that much as Justin Hill summarizes for the new edition of Best Evidence in Brief:

A recent meta-analysis conducted by Sumudu R. Mallawaarachchi and colleagues analyzed studies focused on the association of smartphone and tablet usage with psychosocial, cognitive, and sleep-related developmental factors in early childhood. The meta-analysis was conducted on 19 articles from a variety of countries. The mean ages for the children in these studies ranged from 1.43 years to 5.42 years.

The overall findings of the meta-analysis suggest there is a weak, negative association (r = −.08, p = .001) between smartphone or tablet usage and child-specific factors. However, when individually analyzing these child-specific factors, most do not demonstrate significant associations with smartphone or tablet usage. Psychosocial factors (r = −.07, p = .115), self-regulation (r = −.03, p = .65), cognitive factors (r = −.07, p = .14), language development (r = −.09, p = .09), and executive function (r = −.09, p = .14) all show non-significant associations with smartphone or tablet usage. The only significant individual association was found between smartphone or tablet usage and sleep outcomes (r = −.15, p < .001). A separate analysis, which focused on parental perceptions of issues with smartphone or tablet usage, demonstrated a stronger, negative association with child-specific factors (r = −.31, p = .001).

The findings presented in this meta-analysis suggest that increased smartphone or tablet usage in early childhood is negatively associated with child-specific outcomes. However, this association is weak, and the individual factors negatively associated with smartphone or tablet usage is less clear. More research is still needed to better understand this association and to potentially refine recommendations for early childhood smartphone or tablet usage.