Anxiety about the nation’s children, which is at a steady simmer in the best of times, boiled over in the summer of 2020, when the digital soldiers of QAnon occupied the otherwise innocuous hashtag #SaveTheChildren. Around the same time, major social-media platforms had started blocking overt QAnon accounts and hashtags. From their new beachhead, the digital soldiers were able to disseminate a cascade of false information about child trafficking on Instagram and Facebook: Children were being trafficked on the hospital ship USNS Comfort, then docked in New York City, and through tunnels underneath Central Park. As outrageous as these allegations...