I have nephews who are 4 & 6, and I've explained my job to them when they asked. They know that at my work I create toys and make cool artwork. However, I haven't really gotten into any fundamentals of design concepts with them, other than basic shapes & colors.
Understanding the art of design is crucial to understanding the world and the many subtle clues all around us. Even children can begin to pick up on nuances of design choices in our world and what meaning they have.
If a child is responsive and you can get on their level, you can teach them many ideas . One just needs to be creatively simple in the explanations.