In my mind, the use of symbolism in the short story is to help convey the sense of hollowness that accompanies England in the wake of the bombing in World War II. We see this in a couple of ways. The symbol of the gang's meeting place as a bombed out parking lot is relevant. Their meeting place is a site of destruction. Their sense of communion is only forged at a location where destruction is evident. This is representative of how the gang relates to their world. Such symbolism is seen in T.'s plans to destroy Mr. Thomas' house. The detailed and methodical manner in which the gang takes to blow up is symbolic, as well. The group takes a detailed approach to destruction. This is all they know. The gang of kids is not shown to be able to construct anything. All they do is destroy. This is their sole capacity. I would finally suggest that the destroyed house of Mr. Thomas is symbolic of a time in England that has passed. At the end of the story, Mr. Thomas demands some type of acknowledgement that his home is destroyed. His response from the lorry driver is one of humor as well as the claim that "it's nothing personal." For Mr. Thomas, there is little personal involved here. The reaction he sees after his house has been leveled to the ground is symbolic of the new post- war world in which England enters. This world is one in which there is "nothing personal." The destruction of his house and the callous response of the world in the form of the lorry driver to it is reflective of a setting in which emotional attachments are lost in favor of modern destruction.
This is a short story in which the act of destruction is of course intensely symbolic as it represents the way in which T. and the other members of the gang have become profoundly psychologically damaged as a result of growing up during the Second World War. One of the most profound examples of symbolism in this story is when T. finds Old Misery's money and chooses to burn it rather than steal it. The act of burning the money becomes a powerful symbol of T.'s extreme detachment from society and his withdrawal from the values of his day. Note what he says to Blackie when he is asked if he hates Old Misery:
"There'd be no fun if I hated him." The last burning note illuminated his brooding face. "All this hate and love," he said, "it's soft, it's hooey. There's only things, Blackie," and he looked round the room crowded with the unfamiliar shadows of half things, broken things, former things.
The way in which T. burns the money and his speech afterwards is symbolic of the complete emotional disengagement as referenced to his description of "hate and love" as being "soft" and "hooey." The use of the burning of the money as a symbol thus allows Greene to explore the psychological impact of growing up during a war on children, and the chilling results it can produce.