Teach your Students (interested in Science) the law of conservation of Energy. This is how -
When a vehicle or an object is at a height(ramp top in this case), it is said to have some potential energy when compared to a similar object at a lower level. As the object starts rolling down to that lower level, this stored potential energy is converted to kinetic energy which is the energy with which it travels or rolls. The conversion is not equal though. There's always some energy spent due to friction. Thus, the potential energy is converted to kinetic energy and frictional energy together.
Another thing that can be learnt is the coefiicient of friction. Every surface has a resistance to motion at a varying degree. By recording the distance travelled by the hotwheels for different surfaces, we can differentiate between smooth and rough surfaces easily.