Last week, President Obama signed an executive order aimed at childhood obesity and placed his wife, Michelle, in charge of the effort. Apparently bored with giving first lady-type speeches, and digging up turnips and spinach in the White House garden, Mrs. Obama will head up a national campaign tabbed “Let’s Move.” To be clear, fat kids are nothing to be celebrated. Obesity among children is connected to all sorts of aliments, including heart disease, high blood pressure, cancer, asthma, and diabetes. There is little disagreement about this. But beyond encouraging healthy behavior, what is the government’s role in decisions that...