I think that one aspect of Lawson's story that makes it humorous is that it shows the deliverance of justice in the most bizarre of ways.  Tommy's exploits with the explosive is seen in a different light when he encounters the "vicious yellow mongrel cattle-dog."  This dog had bullied and intimidated Tommy before and, following his character, does so again.  Only this time, when he bothers Tommy, the dog wants what Tommy has, which is a bomb.  I think that the humor lies in the fact that Tommy is intimidated, but this results in a form of justice as the cattle- dog and his cohorts are blown to bits as a result of their bullying Tommy.  Had Tommy been left alone, the story becomes sad in that presumably Tommy would have been ripped apart by the bomb.  Yet, in the fact that Tommy had been bullied, his life was spared and the bullies actually suffered the greatest of punishments.  There is humor in something bad happening to a force of malevolence. It lies in the fact that justice, in some form, has been delivered. It lies in the fact that there is some level of justice at the cost of the perpetrators. As the explosion is described, the humor lies in how those who intimidated Tommy were punished, and how he, "the great, idiotic mongrel retriever," actually won without even knowing it. I think that this is where the humor of the story lies.