We had some Pop Rocks here, and they, like pop, are made with carbon dioxide (Pop Rocks FAQ). I know Bria has had Pop Rocks before, but she didn't seem to remember them. First I let her and Nadia each eat some (of course).
Then I had them put some in a dish of water to see what would happen. They could see that as the Pop Rocks dissolved in the water gas was released in the form of bubbles. They could also hear the crackling of the gas breaking out of the candy.
Next to actually show them the gas I decided to combine Pop Rocks and Pepsi to blow up a balloon. I did one with baking soda and vinegar as well for comparison. I started by putting baking soda in one balloon and Pop Rocks in another. I added vinegar to one bottle and Pepsi to another. I didn't use any exact amounts, but you definitely don't need much vinegar and baking soda! Here is a site with more exact amounts for your vinegar and baking soda (Balloon Blow Up).
I put each balloon on the top of their respective bottle and had each girl hold the end of a balloon. When we were ready I had them both hold the balloon up straight to dump the contents into the bottle.
Both balloons inflated although you can see the Pop Rocks balloon didn't get nearly as big as the baking soda balloon.