Chemistry has played an important part in my life as Chemistry major and high school teacher. While particle accelerators, discovering elements, and Higgs Bosons mean zero to my three-year-old son (editor's note: um, we're with your three-year old!), he sure does love balloons and moving objects.
This demonstration of a penny spinner--which bears remarkable resemblance to a particle accelerator (aka, an "atom smasher") is fun for high-schoolers, pre-schoolers and grownups, alike. No actual knowledge of chemistry is required. In fact, if memories of high school chemistry make you shudder (or too hazy to even recall), just call this a penny spinner--your kid will still think it's the coolest invention in town!
Then blow up the balloon and tie off for your child. Holding the balloon by the sides, shake it left and right. When you get the right rhythm the penny will rise. B has the impulse to shake with a lot of vigor and in every direction. He does get the penny moving in a circle eventually. As he rocks the balloon more, the penny gets more energetic and climbs higher.
B played with his accelerator all day! But unfortunately it was mighty deflated the next morning. ( I highly recommend buying more than one clear balloon so the fun and experiments can continue.)
Now I can dream that I inspired my son. He will call me from Swtizerland to say, "Mom, I found a new sub-atomic particle. We're naming it the mommytron." Then I'll just have to hope he doesn't find the anti-mommytron.