Modern software is an organic soup of interacting components. To plant in your students the sense of perceiving complex system with many interacting components, I would start with the great StarLogo TNG software, which allows children to program 3D simulations & games, almost without any training. After that, I would recommend the best programming language out there (IMHO of course), Python, which is easy to learn, built on good OOP principles & powerful for any task at hand. For letting your students really use what they've learned, teach them also Web development, e.g., with Django, & teach them how to deploy Web applications to the Google AppEngine cloud. By experimenting with this & building useful stuff that is available for all, they'll truly learn what's programming all about.