Teaching My Kids to Be Fit, Not Skinny

Kids Swimming Photo by Kim Silkebækken
Although my kids are only 3.5 and 6.5 years old, I already worry about their perceptions of body image. My son has asked, "Does this make me look fat?" When I asked him where he heard that, he said Jamie Hyneman from The Mythbusters once said it. He's also learned to say, "I feel sexy!" from Jamie. He doesn't understand yet what these phrases mean, and it seems innocuous, but I know that before I know it, they're going to be in middle school, and they will be much more aware of and more concerned about their appearance.
I want to kick myself every time I tell my daughter that she looks "pretty." Then I wonder if I'm overreacting. I find myself telling her, "You know, princesses aren't just pretty. They're courageous, kind, and generous." She's usually too busy dancing around to pay any attention though.
Although Kaylee does enjoy wearing poofy dresses, I'm grateful she also still wears pants and T-shirts with them. She is willing to get dirty, which is a big relief to me. It would bother me a lot if she refused to do some activities because she didn't want to get her dress dirty. To me, that kind of "princess" is much worse than dressing like a Disney princess. Kaylee at least is still active, and not trying to act like a porcelain doll or something.
I've noticed that Kaylee only has one pair of pants with pockets. Is that a "girl" thing, or is it just that preschoolers' clothes don't have pockets?
I'm very careful not to remark that I feel fat or that I feel bad about my body. I try to remind my kids that people's bodies come in many different shapes and sizes, and they're all okay.
Zach has already said that he thought his uncle was "lazy" because he has a big belly, and he's always napping. My brother-in-law is a pretty active guy, and he does tend to nod off when he's just hanging out on the couch because it's much less stimulating than when he's working in his wood shop. When I asked Zach where he'd gotten the idea of being "lazy," he said it was from some Thomas the Tank Engine episode. Clearly my kids watch too much TV.
I'm hoping that emphasizing eating healthy and exercising will teach my kids about the importance of being fit and strong rather than having a particular body shape, size, or weight.
I'm terrified of either or both of my kids developing an eating disorder when they're teenagers, since so many teens do. If they develop depression or anxiety, which runs in my family, they're probably more susceptible to having eating issues.
I don't know how much of what I do now makes a difference later, but I'm hoping that I'm laying a solid foundation of body acceptance that will last into adulthood. Thankfully, both kids are quite active. Kaylee loves dance and gymnastics, Zach loves doing Wushu (a Chinese martial art), and they both enjoy swimming.
What do you do to encourage your kids to accept and love their bodies?