How is a loss of hope and innocence shown?

Mr. Thomas's house symbolizes the hope of a better world amid the ruins of post-war London. But the members of the Wormsley Common gang have been too brutalized by their experiences of growing up in poverty and neglect to realize this. To them, there's never been any hope. They are children, but they are far from innocent; they've been forced to grow up too quickly by their harsh environment and upbringing.
Trevor's situation is different. Coming from a middle-class background, with all the trappings of money and social respectability, he really should know better. Like his fellow gang members he has no innocence, but the difference is that he's lost his, whereas the other boys never had any to begin with. Trevor's sudden fall in social status has brutally robbed him of a life of ease and comfort. Having lost all hope and innocence, T—as he now prefers to be called—is filled with nihilistic rage and despair. He sees no hope for the future. His lack of self-respect is mirrored by his lack of respect for other people and their property. And so he eggs on his new friends to reduce Mr. Thomas's house to rubble.

One of Greene's strengths in "The Destructors" is to show young people as devoid of hope and promise. Greene challenges the traditional expectations for young people by showing them to be incapable of finding something substantive in the world around them and within themselves. The hollowed out parking lot where the gang meets goes very far in showcasing their prospects and the way in which they view the world.
Greene's depiction of young people is significant. It speaks of a world in which traditional structures are absent. In its place is a world where there is not anything in way of guidance and understanding. This experience is both external and internal. It is one in which the emptiness of the world around them has been replicated within their own subjective frames of reference.
The ending in which the gang abandons their plan of destroying the house and leaving Mr. Thomas in the bathroom to be found by someone else is reflective of a reality in which young people lack hope and innocence both in their worlds and in themselves.