Structured Exercise For Toddlers - Yes Or No?

When I think of the need to be fit for health reasons, I always think of my husband and myself, on how we are reaching that certain age, on how our bodies are not as resilient as they were when we were twenty, on the fact that maintaining a pain free life for me now means absolutely no slacking on the exercise and fitness front, and the rest of the thoughts most fitness aware people will have.
Although I have 2 small children, I have not thought at all about them in terms of exercise, because as I see it now, the spend a lot of time at home and at there daycare running around and jumping and falling and dancing, and doing all of the physical stuff kids tend to do.
Kara from Mama Sweat on the other hand has a list of books for kid fitness, and tells about how her kids and her do yoga together - via a yoga book for kids.
So what do you think - am I being to casual about my children’s fitness, taking it for granted, and should I start ingraining in them the love for exercises as a set aside time, a habit, like brushing teeth and showering?