“Screen time” used to mean sitting around and watching TV. For kids, too much time in front of the TV is linked to obesity, as well as trouble in school. But screen time doesn’t refer just to television anymore. Smartphones, tablets, and other handheld devices are now everywhere. These new screens pose new challenges for parents, says Dr. Jenny Radesky, a pediatrician at the University of Michigan. “There’s so much content available now on something a child can carry from room to room,” she explains. Almost all this new technology was designed for adults, not children, Radesky says. For example, a recent study by her team analyzed ads on YouTube channels marketed to kids. They found that more than half featured some age inappropriate ads, such as those for violent video games. Parents may find it hard to know how much time their kids really spend on handheld devices. Another …