K.W.
Please *don't* teach children about stranger danger. Read Lenore Skenazy's book "Free-Range Kids" for all the statistics to back it up.
Long story short: the people most likely to harm your children are people they know. The people most likely to help your children are strangers. Children have died because they hid from rescuers, too afraid of "strangers" to allow anyone to help them.
I teach my kids that groups are generally safer than isolation. I teach them that I should *always* know where they are and who they're with. Anyone who wants them to go to a secret place and not tell anyone probably does not have their best interests in mind. This is true whether it is a stranger, a relative, a teacher, or another child.
I also encourage kids to talk to people in public. We practice reading body language and getting some experience of who is open and friendly vs. who is erratic and/or cranky. These are skills they need to practice. These are skills that serve them well. If they are ever lost, injured, or scared, they need to be able to identify who is most likely to be helpful, walk up to them, and ask for help.
This means that the next time they get lost at the department store, they are comfortable enough to go up to an adult and say "I lost my mommy. Will you help me find her?" instead of hiding in the middle of a clothing rack.
I also encourage my kids to trust their instincts and be willing to challenge adults if necessary. If they feel that someone is unsafe or mean, they are trained and willing to go to another adult for help.
I will note that kids who are always willing to go to a higher authority can be a bit of a pain. My 6 year old called 911 to complain to the police that I was making him do chores! We had to clarify that he could complain to the police only if there was a *safety* problem. But it was all worked out. And I was glad to know that he was willing and able to call 911 if necessary.
The only time we discuss "strangers" is when we discuss courtesy. We give strangers more physical space and are always extra-polite with people we don't know. This is because most strangers feel uncomfortable when a three year old walks up to them and says "That's such a pretty purse. Can you show me what you have inside? How much money are you carrying? Do you have any credit cards? Can I count them?" I also teach my kids not to snuggle with strangers, because it usually makes them very uncomfortable. Though my primary goal in this is simply courtesy, it also helps kids get used to the idea of appropriate physical and emotional boundaries with strangers. In this way, kids learn to recognize that overly friendly behavior from an adult is odd, and worthy of mentioning to another adult.
So a summary:
1. No secrets.
2. Be willing to talk to people and ask for help.
3. If an adult or kid is making you feel weird, uncomfortable, or unsafe, find another adult and ask for help.
4. Give people you don't know extra space, because getting too close tends to make people uncomfortable.