McDonald's playgrounds were a sales team, and every kid in the ball pit was working for free. Now an app with 210 million users does the same job, so the slides had to go. Ad people have a name for what a six-year-old does in the back seat when he spots the golden arches. They call it pester power. A kid has no money and can't drive, but he can wear a parent down in about four minutes, and McDonald's built the best pester machine America ever saw. Birthday parties pulled in the whole class at…