Project 1, Concept 2
This top operates on a few different physical principals- Rotational moment of inertia and centrifugal force. The child will fill the top with water, pull the cord to make it spin, and then watch it go. As it spins, the centrifugal force will push the water up against the sides and force it out the holes that are there just for that reason. As the water drains out, the top should initially spin faster due to the lessened weight. (This part, of course, is in a theoretical perfect universe).
This product is relatively safe. It will be made out of plastic, much like that of a water gun, with small holes drilled along the top and sides to allow the water out. The cord is just a simple string that winds up. There are no small parts for children to swallow, and it costs very little to construct.