I had to do something like this a while ago, and I think you're going in the wrong direction: you need to make it more relevant. The main contact 8-year-olds have with computers today is video games and computer games, so the "Our friend the Univac" lecture from the 1960's is not going to mean much to them.
I'd start with "Do you realize every web site you use is a computer somewhere?" Then describe how someone has to actually figure out every graphic, every animation, every piece of text that shows up. Make it visual: bring pictures from "The Incredibles" and talk about how artists and programmers collaborated to create the movie.
I do like your "cashier" example. You might want to make it like a game: "Joey will be the cashier, and Mary will be the scanner. Sarah, pass your item by the scanner. Now Joey has to go look up the price." And so on.
Most important piece of advice: have fun with it!