How does Greene employ tension between the narrator's tone and the story's plot to show the recklessness of the boys in "The Destructors"?
Very interesting question, and that is because you could consider that the boys aren't actually reckless in their actions. In fact, the opposite could be said to be true - they are very careful and methodical in planning, mobilising and organising themselves to bring down Old Misery's house. This is not the random action of a group of youths, but a carefully pre-meditated and organised action that belies the word "reckless". Consider how T. mobilises the group:
T. was giving his orders with decision: It was as though this plan had been with him all his life, pondered through the seasons, now in his fifteenth year crystallized with the pain of puberty.
In fact, the gang seem to resemble more of a workforce than a disreputable group of disillusioned youth - they are all "punctual" on the first day of the "job" and they show they are able to work together and divide into different groups to get the job done. There is clear direction and purpose that unites the boys, allowing them to find meaning and value in their act of destruction, in sharp contrast to their former activities of trying to gain free fares on buses, for example.
To consider the narrator's tone, it is interesting that T. is repeatedly presented as dispassionate - he neither loves nor hates, but is a nihilist in his outlook. He finds pleasure only in destruction, yet the narrator does not seem to condemn him for this - rather he sees him as a product of the post-war setting - a world where the only reality children like T. have ever known is war and its destructive aftermath.