Facebook has notified parents and corrected a technical error that permitted thousands of children using the Messenger Kids app to join group chats not approved by their parents. The app lets children between 6 and 12 years old message and video chat with family and friends who their parents approve. It's unclear how long the flaw existed. The app has been controversial since its launch in December 2017, and child advocacy groups have repeatedly urged Facebook to shut down the app, arguing it violates a federal law aimed at protecting a child's online privacy.