This could be the Coolest science toy ever; check out that video! I know the kids are going to like this kind of thing. You can download the software (it’s beta, so it might not work perfectly; it’s also available only for Windows and Linux, so Mac users have to wait) or find out more here: Phun.
It’s described as a "2D physics sandbox", which means you can create your own objects, give them attributes, and watch them behave in accord with the laws of physics. Everything from dropping a rock on a coyote to building engines and vehicles, as long as they can fit into two dimensions and you can draw them.
As soon as I have a few minutes, I’m going to try this out myself. Stay tuned!