The author chose to tell "The Umbrella Man" from the point of view of a twelve-year-old girl because what happens seems innocuous and humorous as seen through her innocent eyes. The story opens with this girl introducing herself.
I’M GOING TO TELL YOU about a funny thing that happened to my mother and me yesterday evening. I am twelve years old and I’m a girl.
She tells the reader her age and introduces her narrative as "a funny thing." So everything that happens to the girl and her mother will seem to be funny to the girl. Otherwise, the incident might not seem particularly funny. After all, this old man is stealing umbrellas in order to drink a lot of straight whiskey. The owner of the umbrella which the old man sells to the girl's mother would not think it was funny. The umbrella was supposedly worth twenty pounds, which was equivalent to about a hundred American dollars at that time and equivalent to ten or twenty times as much in today's dollars. But perhaps we shouldn't feel sorry for any man who could afford such an expensive umbrella.
The narrator's mother doesn’t think what happened to her is funny. She is stuck with a stolen umbrella and probably can’t decide what to do with it. She would have no way of returning it to the owner, and she wouldn’t even know which pub it had been stolen from. It was certainly not taken from the pub where they see the man downing a triple-shot of whiskey.
What did the twelve-year-old narrator think was so funny? She is probably amused at the realization that such things happen in the world. She is young and doesn’t get to town very often. She is also secretly amused by the way her mother was so completely taken in by the old man’s story. The mother tells the narrator:
He was a gentleman. I’m very pleased I was able to help him.’
‘Yes, mummy,’ I said.
‘A real gentleman,’ she went on. ‘Wealthy, too, otherwise he wouldn’t have had a silk umbrella. I shouldn’t be surprised if he isn’t a titled person. Sir Harry Goldsworthy or something like that.’
‘This will be a very good lesson to you,’ she went on. ‘Never rush things. Always take your time when you are summing someone up. Then you’ll never make mistakes.’
The girl is delighted with the fact that they are both playing detectives when they follow the umbrella man through the rain and see what he is really up to. There can be no doubt that her mother has bought a stolen umbrella from a con man. The girl is outwardly very well-behaved, as she has been taught to be; but underneath she has the same sort of impish character that is depicted in the young lady who tells Framton Nuttel the ghost story in Saki's "The Open Window." The narrator is obedient. She keeps saying, "Yes, mummy." But she is a bit rebellious and competitive. She makes a point of saying in the opening paragraph:
My mother is thirty-four but I am nearly as tall as her already.
We can see more in this story than the young narrator. We can see that the real point of the story is the narrator's enjoyment of her mother's self-deception and subsequent embarrassment. The girl tells us that this happened only yesterday, so her mother will still be wondering what to do with the silk umbrella she bought for only one pound. She knows knows that her daughter knows the umbrella is "hot." If she keeps it and uses it on rainy days, her daughter will know her mother has a stolen umbrella. How will that set with all the morality the mother has been trying to instill in her daughter for the past twelve years?