I do not have the answer to that, but I do know that the American Pediatric Association still recommends that parents limit screen time. In fact, the limit is 1 hour per day for preschool age children. As a mom myself, I know that the screen is very appealing for children and often an easy distraction for busy parents. So, if they are on the screens, what are they doing? Is it meaningful? High quality?
Here is an article from PBS Parents that should help you make solid choices for your child when he or she does go on that tablet or phone.