At five years old, Charles is understandably far less polished and together than his older sister – he's a little kid, and he convincingly does all the things little kids do, like tell bad jokes (over and over again) and run around wreaking havoc. He's quite exuberant, which may have to do with the fact that Esmé and the grownups that take care of them have made sure that he's sheltered from the more terrible aspects of life. For example, Esmé may or may not have told Charles that their father was killed – in her words, "s-l-a-i-n" – in the war. He's still in a state of total innocence and blissful ignorance, and as a result, he embodies the kind of carefree joy of childhood.
It's this genuine sense of innocence that appeals to the Narrator; after he unintentionally salts Charles's game by ruining the punch line of his riddle, the Narrator is truly concerned about the little boy's feelings. It seems that he doesn't want to do anything to ruin Charles's youthful exuberance. Fortunately, Charles isn't easily dismayed, and he departs the story with his childish joy intact.