Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Nostalgic, Overawed
The Great Brain blurs the lines between character and narrator. The character of J.D. is a seven-to-eight-year-old boy, but the narrator is an adult looking back at his childhood. Well, except when he is a kid living in the moment—the narrator moves between his child and adult perspectives without warning, and it's often a subtle switch that's easy to miss.
That said, in general, the adult J.D. feels very nostalgic for his childhood. Just about everything is perfect—Mamma, Papa, Adenville:
Mamma's blond hair was piled high in braids on her head. The sunshine coming through the kitchen window and striking Mamma's head made her hair look like golden sunlight. (1.25)
As for the child narrator, he's in awe of his elder brother, Tom, the Great Brain, while also being somewhat annoyed by him:
I couldn't help feeling before I fell asleep that somehow and in some way I was going to end up with the short end of the deal. And, oh, how I wished I had a great brain like my brother's so I could figure it out. (3.221)
Taken together, we get the sense that for all his wiliness, Tom is ultimately harmless—he's adored both in the moment and in retrospect, making it clear his scheming never goes too far.