The principle this toy is based on is centripetal force, as well as spring forces. It can be observed by the child when they spin the top and notice that the spheres that were originally in the center of the top have moved out towards the edge of the top. As the top slowed down, the spring attached to each sphere would pull it back to the center. The spheres would move more or less depending on how fast the child spun the top. 

This toy would be cheap to make because it is a relatively small toy and doesn't require any complicated parts. It would be fairly durable, aside from the rods that extend out past the edge of the top which allow greater motion of the spheres. Depending on the thickness and material of the rods, they could be broken by rough usage. It would be a safe toy for children unless parts broke off or unless it was small enough to be choked on.