My mom teaches pre-k and every year does a demo using eggs and a truck. She has a toy truck that can fit two eggs in the front. All the kiddos get an egg (I think they are hard-boiled...otherwise it seems this would be WAY too messy.
) So each kid gets an egg and they get to color the egg or draw a face on it or whatever. They make it *their* egg in whatever way they want to.
Then my mom gets two of the kiddos to go first and she puts their eggs in the truck. She asks them if they want to buckle their eggs in or not. If they choose to buckle the eggs in then she uses some regular old scotch tape to secure the eggs. At this point she'll usually ask the kids some questions about what they think will happen, but for you it might be better to get right into the action. Then she (or maybe she lets one of the kids) propels the truck into a barricade a few feet away. The eggs that aren't secured will often fly out and break. The kids catch on pretty fast, and if no one ever wants to not buckle their egg in my mom will pull out an egg she previously colored to make sure the point of the demo isn't lost.
Last year my mom was working on some certification that required submitting videos of her teaching and so I helped her transfer a video to dvd of her class doing this demo. Even I thought it was interesting and the kids seem pretty engrossed. (I still remember my mom saying "Oh no! Look what happened to Maria's egg" and all the kids gasping!) It does take a bit of preparation and might take too much time for your particular event. Plus its not really a proper carseat demo, though it is very age-appropriate for 4-year olds.