My approach is to explain how I started my company, the successes and failures of my previous ventures and the fact that there is nothing in the world more fulfilling than being able to rely solely on oneself, being able to start something and watching it grow. I'll then give them my number and tell them that if any of them ever has an idea they want to run with, they should never hesitate to call me at anytime and I will help them in every way possible to see if we can turn the idea into something.
Also i'll tell them to spend some time alone every once in a while to think about any issues or problems they've come across in their daily lives and see if they can come up with simple ways to solve those problems. The best ideas can often come from thinking about solutions to fix problems (big or small) that we face every day.
I also want to let them know to spend as little time watching television as possible but not sure how to approach that.
I haven't fully fleshed out the speech yet and it may be overly idealistic but I think its better than giving a little pep talk then booking out of there.