Graham Greene includes the detail about the nine year old boy for several reasons. Greene uses it to remind the reader that these are not hardened criminals in the story taking down an old man's house, but rather children. His use of this detail highlights their youth and makes a point about this gang. At times in the story, especially consdering the speed and efficiency of the demolition, it becomes easy for the reader to forget that the perpetrators are in fact children.
Moreover, Greene uses the detail to explain the gulf between 'T' and the other children. Trevor is much older than they are, which partly explains the ease of his accension into the leadership role, but also is sort of pitiful, that Trevor can not find boys his age to play with who will accept him.