Depends on the age of the kids. You'll most probably cannot compete with their game console, so forget the 3D graphics ;-) As already suggested playing and doing experiments - something they can participate should awake their interest. Maybe you can type a tiny program, show them how it works, and let them play with it? Then add some more functionality, maybe as a kind of game?

Some ideas, mainly for younger kids:

• let the computer guess a number between 1 and "impressively high" - lower/higher, etc...
• let the kids enter their initials, the computer prints a greeting with the full name...(how can it know that?)
• do you remember the Apple ][ demo program Animals?
• the CPAN Games directory...
• simple encryption/decryption routine (1=A, 2=B, ... type)...
• randomly generated sentences; let kids chose the words...
• a simple calculator using eval...
• Fibonacci? Faculty!
• a mathematical game of the kind add 3 to the year you were born, subtract ... bla bla ... minus size of your shoes ... finally print their age...
• Turtlegraphics...
• remote controlling something... (if you have two PCs)..
• ...

Don't know if that really is what you expected, but a pre-nerd in the right age will become a dedicated follower ;-)