Kids are taught to be polite and respectful, but when it comes to their safety, tell them that it is okay to be loud.
If someone approaches and they feel unsafe, teach them to create a wall with their body by putting their hands in front of them and yelling "Stop. Leave. Help."
Attackers want privacy and control. Yelling takes those things away and alerts people your child does not feel safe.
Practice this over and over with your children so that they do not have to think about what to do in a threatening situation.