Kids can get flustered when someone knocks and an adult isn’t nearby. They might freeze, stay silent, or, on the other extreme, open the door immediately. A simple script gives them a safe middle ground.
Something like: “Who is it?”, followed by “Please wait, I’ll call my parents,” is enough. They don’t have to decide whether to let someone in or explain complicated things. Their job is only to ask, listen, and then fetch an adult.
Practising this script a few times makes it automatic. In a real moment, they won’t need to invent a response under pressure. It takes some of the fear and confusion away and gives you, as an adult, a bit more control over who actually interacts at the door.