Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Deadpan, comic, conversational; and then serious, grim, psychological
"For Esmé" allows Salinger to show off his mad skills with regards to tone – the two section of the story are like day and night. The first half is lighthearted, dryly comic, and chatty; the first person narrator's interaction with the two children has a terrific mock-seriousness (combined with a sense of real, earnest actual seriousness) that's incredibly fun to read, as well as touching. Salinger is always at his best in dialogue-heavy scenes, and his naturally conversational rhythm captures the alternately hilarious and sweet exchange between the narrator, Esmé, and Charles.
The second half of the story is a horse of a completely different color. It's cynical, grim, wartime drama, and Salinger shifts his tone accordingly. The move of the narration into the third person separates us and alienates us from our protagonist, who we now see at a distance, and the overall tone is much darker and totally serious. There's dialogue here, too, but it's far from lighthearted – it's cynical, sarcastic, and has an edge of cruelty that's particularly distressing after the innocent, kindhearted first half of the story. This marked contrast in tone is fundamentally important in our reading of the story as a whole.