I have an 8 year old too. Codes and ciphers are real popular with her and her classmates right now. About a month ago I took her to the library and she checked out every book she could find, and she's spent hours creating her own substitution ciphers.

A few days ago she asked me if I could teach her how to "hack". I thought this was a great opportunity, so (after I explained the difference between "hack" and "crack") we sat down with paper and pencil and worked out what would have to happen to encrypt and decrypt a simple message. Then we sat down at the keyboard together and duplicated rot13, and she loved it.

It gave me an opportunity to talk about how characters are displayed, chr and ord, then tr. Since then, she's asked probably 10 times what else she could learn!

Our next project is printing triangles. You know the type...

X
XX
XXX
XXXX
[download]

Bottom line, I'd start with something fun that catches their attention as well as gives you a chance to explain some of the basics - I/O, conditionals, simple calculations, character manipulation, graphics, flow control, etc. If your presentation entertains, you'll engage more of the kids.

Whatever you decide to do, please post the results back here so we know how it turned out.