Drawing a toy car is a very good exercise to practice how to draw mechanical objects.
Imagine a car is an objects with 2 boxes stacking together. A smaller box sits on the top of a bigger one. Make sure the angle and the proportion are correct. Refine the outline of the cars and draw the details. Put the shading on the cars and don't forget the shadows!
Encourage students to try drawing with a real object and create their own scenes. Design the car with toy cars.