There’s an old saying that when all you have is a hammer everything looks like a nail. The government version is this: When all you have is the ability to regulate, the solution to every problem is more regulation: Citing an epidemic of childhood obesity, regulators are taking aim at a range of tactics used to market foods high in sugar, fat or salt to children, including the use of cartoon characters like Toucan Sam, the brightly colored Froot Loops pitchman, who appears in television commercials and online games as well as on cereal boxes. The story goes on to...