We can state the central theme in a couple of ways, but I think the main idea of this story is this: Extreme circumstances can drive people to act in ways that do not fit their usual characters.
The boys in the gang had lost nearly everything to the bombings of WWII. They had lost their homes or feared for them daily. Their neighborhood is described as having "...suffered from the blast of the bomb and the side walls were supported on wooden struts. A smaller bomb and some incendiaries had fallen beyond, so that the house stuck up like a jagged tooth...." They had lost their childhood. They no longer felt safe in their world.
Thus, they were driven to petty mischief which is normal for most boys, but when they meet Trevor and agree to his destructive plan, they are truly acting out of character. Trevor's desire to wreck Old Misery's home was driven by jealousy, and he was able to draw the others in.
It is doubtful that any of the kids would have joined in the destruction if they were not disillusioned by the adult world to begin with. They were victims of destruction themselves, so their destructive acts, while not condoned, are at least understandable.