This is an excellent idea. By using very simple encryption as your example - eg the Caesar cipher or the Vigenere cipher - you should be able to teach them something useful in twenty minutes. Start by teaching the Caesar cipher, then show how quickly and easily it can be broken using brute force or trivial frequency analysis, then introduce the Vigenere cipher and show that it doesn't fall to either attack. At least, it doesn't when you're working with a pen on a whiteboard.

And most importantly - don't teach encryption, or ciphers, or algorithms, or programming. Teach them how to pass secret messages to their friends that their teachers won't be able to read.

Details about the Vigenere cipher are here.