The young teens who form Blackie's gang in theory should be at the age where they are beginning the process of losing their innocence and naivety. However, it is clear from their speech and actions that they have already reached this stage of cynical world-weariness. They have become old before their time, and in a very real sense have not been allowed a "proper" childhood due to the rigours and stresses of war. This is a fact they cannot escape, for they are literally surrounded by the detritus of wartime, meeting in a car park which was the site of the last bomb of the first blitz and surrounded by bombed buildings.
Interestingly they try to impose an "order" on to their surroundings but it is obvious that this order is merely destruction - as they themselves have been "destroyed" by the war, now they are only able to mimic these acts of destruction on their environment to express their anger and loss.