They are also learning about forces. Basically, they're learning about F = ma, which is Newton's second law. Force = mass of object times acceleration. You guys apply a force to start your Hot Wheel on the track, and hopefully the force is enough for the Hot Wheel to complete the track. Some of you are using your hands and others are using the launcher. If you are able to do both, show the students both ways and have them predict which yields the greater acceleration. Run this lesson with each of your cars and explain why you get different results for each car.
Of course, let them play with your tracks, ask them questions about what they've learned, and encourage them to explain back to you what is going on with the track. Ask them about Newton's 2nd and how to apply forces. Basically, you're having them summarize the lesson.