1) You must define the word “stranger” for your child. I would explain to your children that a stranger is anyone who had not been introduced to them by you, by other members of your family, by the teacher, or by someone whom your child knows YOU trust. Always bring it back to you, as you were the person who initially introduced your child to other family members, the teacher, etc. To children, a person stops being a stranger when the person jokes around with them, let’s them pet a dog or look for a lost dog, gives them something, like ice cream. In other words, children think the person is no longer a stranger when they have simply broken the ice with them, and this can happen in a few seconds. Think of it in this way as well: when a child hears the word, “stranger,” they think “strange” like Dracula or Darth Vadar. They do not think the ice cream man is “strange” nor is an ordinary man looking for his lost dog. So you must make them understand YOUR definition of a stranger.
2) Take your child through a number of different hypothetical situations and explain to them how these are all stranger situations and that they need to scream and run away fast. Make them repeat the situation back to you so that you know they understand. They will also be more likely to remember the situation if you make them speak it.
3) Do not assume your child gets it the 1st or 2nd time you tell them about stranger danger. Just like you have to tell them several times to make their bed, brush their teeth, clean up their toys, and not to interrupt adults, etc., you will have to tell them this repeatedly as well.
Regretfully, this is the kind of world we live in. Better to be safe than sorry, as I see it. Educating your child about stranger danger is no different than educating your child about the dangers of running into the street, playing on the railroad tracks, etc.