I do agree with Ania that there are so many other beautiful math subjects that get ignored. Discrete math is wonderful and can be taught on many levels. Set theory, logic, probability, number theory, and graph theory are just a few of the topics that would appeal to elementary kids.

I have mixed feelings about teaching calculus to elementary kids. I think the theory is good--teach kids through problem solving and not through mindless memorization. I also think some elementary kids can grasp the basic concepts of rates of change and limits. However, problem solving in calculus is very dependent on algebra skills, and I'm not sure that many elementary kids would really be able to do it. I think that exploring limits and rates of change (the very basics) without getting into real differentiation and integration (which requires knowledge of algebra and functions) is worth a shot.