Windmill

Principle: This windmill is based upon the principle of energy generation. It will teach kids that motion (angular in this case) can be transformed into electrical.

Interaction: when the child spins the propeller, some small time after it begins to spin, the light will go on. the faster the propeller spins, the brighter the light will illuminate.

Durability: Durability concerns would mainly come from the ability of the propellers to stand up to a child pulling on them. The rest would be fairly solid hard plastics.

Safety: Because of the propeller being thinner and less durable, there would be a choking hazard associated with it  if it were to come loose or break off.  

Manufacturing:   It would be constructed with a base, sensor to detect motion, and light bulb. the base would be 2 pieces and then the generator could rest on top of the base with a cover going over that then the propeller would slip on the axle.  

Cost: The cost to manufacture would be fairly cheap (estimated $2-$4) and so it would probably sell for around $9-$10

Age Recommendation: 4-10 due to choking hazard.

 for construction, I would imagine the casing being 2 pieces with the internals inside and so the cost would be low. depending on the materials, it could be fairly durable and may pose some safety risk to smaller children if it were to break. Because of this, the recommended age would be  around 5 to 10  or so.