A funny thing happened when I banned tablets in my house on weekdays and curtailed their use on weekends. My children, ages 6 and 4, became less cantankerous. They also became happier, more responsive and engaged in more imaginative play. They rediscovered their toys. Outside the home, they became less demanding and better at self-regulating.

Friday, the American Academy of Pediatrics validated my experiment, recommending that children younger than 18 months get zero screen time, and those ages 2 to 5 be limited to one hour a day—half of its prior recommendation. The group recommended that the hour be “high quality programming” that parents watch with their children.

The academy doesn’t set limits for older children, but suggests curtailing screen time before bedtime and when it conflicts with healthy activities.

Most parents haven’t been listening. Mobile devices—tablets, smartphones and the like—in the hands of children are a big business. Time spent in apps from the “family” category on the Google Play store doubled in the past year, according to app-tracker App Annie. Children ages 2 to 11 watch an average of 4 ½ hours a day of recorded programming. And more than 50% of Netflix Inc. accounts world-wide watch some form of children’s content, a spokeswoman says.

We have been conducting a social experiment on our children since the arrival of the smartphone a decade ago and the tablet soon after. A rich library links too much television for children to ill effects ranging from obesity to attention disorders. But there are few studies examining children using tablets and smartphones.