More than half of the young children in the U.S. now have access to an iPad, iPhone or similar touch-screen device. For parents, their children's love of these devices raises a lot of questions.

Kids for years have sat too close to the television for too long or played hours of Madden on family room game players. But pediatric neuroscientists and researchers who have studied the effects of screen-time on children suggest the iPad is a different beast.

A...