Posts about a couple's "secret word" (meaning "Get out of Dodge!" or the like) reminded me of something our family used... and still would if the situation presented itself:
We might someday need to send someone to my daughter to pick her up or deliver a critical message. Before going with them (or believing the message was from us), she would need to verify that we were indeed the source. To do this we have a special code word she must hear before she will believe the story is legit. Classic tales of abductions start with "Your parents have been hurt and they want me to bring you to them!"... but without the secret word she wouldn't buy it. (It's less of a concern now with her at 20 having Krav Maga and pistol training, but still...)
We picked a pair of words that would create a strong mental image so she could memorize them at a young age (for example, "ballerina cow" or "flying trashcan").
Might be useful for other parents, too.