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I wrote about this on my blog a few weeks back, pondering the same conundrum. And the conclusion I came to is...
Flash.
Yep, that bane of many web surfer's lives is ideal for kids to learn to program with.
You can do simple graphic animations quickly and easily, then if you want to delve deeper, say, to program a simple game, you can, and you can do it using a language (ActionScript) that is similar in form and function to many of the most commonly-used programming languages in the workplace (C++, Javascript, Java, etc, not to mention Flash itself), and it's an easy way into understanding object-oriented principles. Plus, it's relatively simple to post the fruits of your labours to your MySpace page for all your friends to admire (since most computer-literate kids these days are all on MySpace).
Macromedia should take an older version of Flash (maybe v6), and make it freeware, possibly even brokering a deal with Microsoft or Apple to include it in their respective OS installations.
Just my twopenneth worth.
Russell.