Thanks for the feedback everyone. I really like what I see over at http://scratched.media.mit.edu/ so I'm leaning towards this solution with Game Maker as a backup.
I'd like to show them some basic programming concepts like logic statements, looping structures, and basic object oriented stuff relating to methods & attributes. What I want to get away from is throwing a language like C at them and then having to spend 2 hours finding a missing semicolon. Granted, this can be a very real part of the process, but I think our time would be best spent working with a more "visual" code creation tool like Game Maker or Scratch.