Thanks for the responses everyone.
I understand the difficulties with teenagers and peer pressure and all the things I can do to model a sensible approach, what I really don't want to do is entirely cede the cultural argument for personal transportation to the automobile.
When I think of the hours I spent with toy cars, the thousands of commercials, the comics and cartoons about cars and the fact that no adults are ever seen on TV or in the movies on bicycles, it's a wonder I ever figured it out (it was actually a motorcycle accident that brought me to cycling as an adult). Just hoping to carve a little more space in my toddlers' heads on the subject.