Not to beat my own drum too loudly (sorry!) but please see TheOpen ReSource Project
for some thoughts.
I disagree that kids in the 8 - 10 range cannot grok programming. Some of our most potent sessions started when kids got an epiphanic moment of understanding the interactive nature of a shell prompt. My 5-year-old can learn lots if I give it to him in small doses.
I would suggest that you make up a simple read-eval-print Perl program that does something simple with the input a child types, such as transforming said input into ig-pay atin-lay. Laughter is important!
You might also have it blow up the output by printing character graphics of the transformed string on the screen, paged with spacebar. You might consider a program for drawing a spirograph image on a Tk canvas, as another possibility. Also, design your demonstration so that there is something simple that they can modify in the program, so that they see immediate results. If your shell and Xterm can print color, use escape sequences, and show them how to change the output by changing a single byte. Plan for three or four different transformations that you can cycle through with simple keyboard changes. Be ready to revert your editor to last-saved, because some of them will
screw up badly. Have both a flowchart and code printout handy for presentation to the adventurous. Most will toss it in the trash, but some will see the parallels and learn a great deal. Paper is only $3.50 a ream; what price inspiration?
One CRITICAL thing to make sure of is that you have everything
ready and have your presentation scripted beforehand. Kids are used to high-bandwidth multitrack shovel-feeding from TV and games, and you can only get a little dose of inspiration in before they get impatient and the moment is lost.