Our son is active because he sees me run and participate in races, thus he wants to be fast like his daddy. Both of our kids think that fast food is junk because they hardly ever get it (maybe two or three times a year at most), and we tell them its junk. During the winter when they are shut in the house so much, I have them do exercises in the house (run the stairs, pushups, lunges, crunches, so on - pretty much a little drill camp for an 8 and 5 year old
). It does them good.
Parents do their kids a huge disservice if they allow their kids to lead sedentary childhoods and eat a bunch of junk. If your obese as a child its very difficult to lose that weight as an adult. I will go so far as to say that its very poor parenting to let your kids become obese. Basically, your shortening their lifespan and hurting their future quality of life by doing so. Really, when you think about it, its every bit as bad as letting them flunk in school.