Trends in child health are in fact worrisome, and scientists welcome a renewed focus on foods and environmental toxins. But vaccines and fluoride are not the cause.Robert F. Kennedy Jr., now President-elect Donald J. Trump’s pick to lead the Department of Health and Human Services, has for years called attention to what he considers an “epidemic” of chronic disease that has left America’s children among the sickest in the developed world. Mr. Kennedy blames environmental toxins and a broken food system. But he also points to some of the most widely acclaimed advances of the last century: fluoridated water and...