LONDON—TikTok, the wildly popular video app, and its Chinese parent ByteDance could face a damages claim worth billions of pounds in London’s High Court over allegations they illegally harvested the private data of millions of European children. Anne Longfield, the former Children’s Commissioner for England and so-called “litigation friend,” or public face, of an anonymous 12-year-old girl leading the class action, said on Wednesday that affected children could receive thousands of pounds each if the claim is successful. Longfield alleged that every child that has used TikTok since May 25, 2018, may have had private personal information illegally collected by ByteDance through TikTok for the benefit of unknown third parties. “Parents and children have a right to know that private information, including phone numbers, physical location, and videos of their children are being illegally collected,” she said, as a website to detail the case goes live. A TikTok representative said …