Yoga is one of my very favorite things to do. I love how it works and stretches my muscles and the tingly relaxed feeling I have afterwards - not to mention killer flexibility. I've been practicing yoga in my home for several years now, and I find it helps tremendously with my joint discomfort as well promoting calmness and mental clarity.
Doing yoga with my kids just feels like a very natural order of things, but I also find other ways to exercise them. And of course, at their age they don't need too much encouragement to get up and run. Check out Karlie doing the downward dog. Very limber!
If you're not sure how to go about practicing yoga with your child, I highly recommend this dvd by Wai Lana, called "Little Yogi's". Any Wai Lana dvd is good, and she's easy on beginners if you're just starting out.
Wai Lana's Little Yogis