Christmas is a time of joy and discovery, especially for children, but young Florence Widdicombe got more discovery than anyone would wish. After buying greeting cards from UK supermarket giant Tesco, the six-year-old began writing out her Christmas greetings. What she found in one card, however, set off an international dispute over China’s prison-camp operations, trade policies, and called into question just how much the West is prepared to overlook in its relations with Beijing: After a 6-year-old girl found a plea for help from Chinese prison laborers inside a Christmas card she was preparing to send to friends,...