I completely agree with you AC. What an amazing opportunity. I wouldn't squander it with facts and theories that a nine year old wouldn't understand.

First of all, your kid is amazing. I love his curiosity about the world. To a nine year old, "imagination" is more important than "theories". I would answer the question this way... Einstein used his imagination to visualize how the universe works. He could imagine time, speed, distance, and scale in a way that enabled him to formulate mathematical models of how these things were related. Based on Einstein's understanding of the universe, inventors have created amazing things like: smart phones, Google maps, and atomic energy. Einstein is famous because he used his imagination to find a new way of looking at the world. Einstein once said, "Imagination is more important than knowledge". Try to remember that when you are bored at school. Remember to imagine.