There's also what it's taking the place of. Even if it's not directly harmful, and I think that's one hugeeee if, it's a massive timesink that they would otherwise spent doing other things. And it's difficult to imagine anything that could be worse except maybe just sitting in front of the TV. Though even there, TV has the benefit of being much more boring, so the kid may be more inclined to voluntarily seek other entertainment; so perhaps even TV isn't worse.