Most likely, the batteries from the toy caused the fish's fins to be eaten away. Batteries contain acid that will decompose things that it comes in contact with. This reminds me of the time that my brother got a hold of some batteries when he was just a baby. Of course, he put them in his mouth, and the acid ate away at the skin on his tounge. I assume that the same thing has happened to your fish. I would recommend keeping a lid of some sort on the tank, or placing it high up so that your grandson can't get to it. I wish you luck.