Thank you ALL for giving your advise and suggestions to this topic; we had a group of 14 kids from various area schools (all grade 9s), showed them a 4 minutes intro (instructional) video from YouTube, then a 3 min. "cool stuff" video with tanks and game design by Massive Black, then finally a video on ancient Rome also including the Google Earth interface.
Then one of our architectural Coop University Students winged a 3D model of a Wii console - she did not tell the kids what was it supposed to be, but they guessed anyway within the first 2 minutes.
Finally, we had all of them try the software for 15-20 minutes; some got bored quickly, but about 1/3 got so much into it that they even printed color prints of their models! Great stuff!!!
again, thanks to All for their inputs
Marek