I might get scolded for this <img src="i/expressions/face-icon-small-blush.gif" border="0"> ! but I took our daughter's vest to her preschool class (with her permission) and let her classmates try it, if they wanted to. She showed them how, first. She was so proud! I have a picture of her demonstrating and it is one of my favorites.
This is dorky, but it was effective for her age group: I drew a set of lungs on posterboard, and then I drew another set of lungs on a different piece and made them "sticky" by putting double stick tape across them. Then I got craft pom poms (I experimented with several sizes for the right effect.) These were the "germs". I sprinkled the pom poms on the normal lungs, and they slid right off. I sprinkled them on the sticky CF lungs and they stuck. So then we had to tap the poster board to get the germs to come off. This was supposed to demonstrate the same concept as to why she had to use the vest. BTW her class raised $3,000!
Also - do you have any old nebs laying around, and maybe some tubing? What if you brought those and some baby dolls, and let the girls pretend to give breathing treatments to the dolls. You know, we take this equipment all for granted but maybe some kids have never seen it. What about M&M's or some other representation of how many meds Taylor has to take?