Thanks for the suggestion. Some time ago I'd looked at Processing for my own use. I hadn't even considered it as a platform for teaching. It would be certainly be interesting to have a kid-oriented programming environment (like that for Scratch) built on top of this.
Here's one that might be worth considering. Kids like to draw. So get them to draw something on either paper then scan it or on the screen with a paint program. Save the image(s) to either a .png or .jpg format.
Then head over to http://ejohn.org/blog/processingjs/ then check out the sprite example ~ http://ejohn.org/apps/processing.js/examples/basic/sprite.ht...Forget python, squeak, ruby. Arc and lisp can wait. Think browser as your OS and Processing as your language. The bonus of creating their own images then given some bits of code to change the numbers. Let them fail a few times. Most important is getting an immediate result.
This is probably the simplest environment that most kids are familiar with, a browser. So use it. There are plenty of examples ~ http://processing.org/learning/basics/ the system is FOSS and x-platform. The example also allows them to create their own "Illustrated Primer" with powerful tools yet still simple to use.