Mandatory reporting means that when someone, like a doctor or a teacher, thinks that a child is being hurt or harmed in some way, they have to tell the people in charge to keep the child safe. This is because adults are responsible for making sure children are safe and happy, and sometimes children can't always speak up for themselves. The people in charge will then investigate and make sure that the child is protected from any danger. It's like telling a grown-up when you see somebody getting hurt or doing something they shouldn't be doing, because grown-ups can help them stay safe.