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DiBona Don't ask to ask, just ask  Comment on   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!