The gold key is a pretty horrific symbol of the lengths the government will go to in order to brainwash children into enlisting in the military. They tell them that this key will get them into heaven if they die at war. This harkens back to ancient religious wars, like the Crusades, where dying a martyr was the best possible thing a boy could do. Although, in reality, all it means is that they died as pawns of the government.
This key is an especially repulsive symbol because it holds absolutely no intrinsic value—it's "a plastic key painted gold" (13.34). The Iranian military couldn't even give kids something of value to lure them into war, something they might be able to melt down for money. Of course, what value does money have to a martyr? You can't spend it when you're in the theoretical halls of heaven, with more virgins for the taking than you know what to do with.