I'll have to echo the other posters and say that teaching the scientific method is important. The scientific method trains and reinforces analytical thinking. Furthermore, knowledge of the scientific method can help people, both as children and later as adults, evaluate reporting on scientific topics. (For example, when they read something on "evolutionary theory" they'll know the definition of "theory" and won't immediately dismiss what's being presented as hearsay.)
Beyond the scientific method, I think that the most important topics to teach children are those that relate to the world around them. Basic astronomy and earth science can help children to understand seasons and weather. Teaching macromolecules helps to inform them about nutrition. Chemistry is good too, as it explains matter itself. Also, learning about simple machines could be extremely useful.