For physical science, I want the children to grasp that they do not grasp gravity, that there's a mysterious force throughout the universe pulling everything together, against another primal force that apparently sent everything flying apart. You know when they get it. They look like the floor just dissolved.
For biology, I want the children to grasp the essence of life, as far as we know it. Life is a crazy quilt of organisms glomming energy from the sun, all living things on Earth descended from other living things, and the environment shapes who we are. Humans were not inevitable. We are all going to die.