Here's a quick one to think about with your kids. If mom was unavailable to come get you and she had to send someone you didn't know to pick you up from school/ball practice/whatever, how would you know they were safe to go with? Well, they'll know the secret code word (or phrase) of course. When we were growing up, we made up a crazy secret code phrase. It is hilarious and nobody could come up with it if they weren't told it, but it would be hard to forget once you heard it. Perfect. Never had to use it. I know you're dying to know what it was, but I can't give it away here because it's a SECRET code word! Come up with some phrase that your kids and you can remember in an emergency situation. Then if anyone comes up to your kids and says their mom needs help and they are suppposed to get in that person's car right now, they'll know to run the other way if they don't hear the code word.
Another place a code word could be used is in an event that requires kids going straight to a safe room in the house or dropping and hiding where they are. Then if it is an intruder at the door, he won't know what you're yelling "BLUE PICKLES" for, but the kids will get the message and do what they need to do.
This code word could also be something you could "text" to your kids if they have cell phones. The key is that the code word enacts a plan that you have already made.
Think up times when your family might need to communicate something without letting the whole world know what you mean (think "Alas, Babylon"), or might need a "password" for security reasons. Come up with some good memorable code words for your family, and by all means, don't tell me what they are! ;)