What evidence is there to prove that Sook is like a child?

Throughout this story, which is a semi-autobiographical portrait of Truman Capote's own childhood, the reader is given suggestions that Buddy's cousin Sook is like a child. In the third paragraph, after Buddy has described what she looks like he explains why she calls him Buddy:  

She calls me Buddy, in memory of a boy who was formerly her best friend. The other Buddy died in the 1880's, when she was still a child. She...

Throughout this story, which is a semi-autobiographical portrait of Truman Capote's own childhood, the reader is given suggestions that Buddy's cousin Sook is like a child. In the third paragraph, after Buddy has described what she looks like he explains why she calls him Buddy:  

She calls me Buddy, in memory of a boy who was formerly her best friend. The other Buddy died in the 1880's, when she was still a child. She is still a child.

When Buddy relates memories of things they've done, he describes their activities as if they are things two children have done. For example, he describes the money they made with a "fun and freak museum" that had a three-legged biddy chicken:  "Every body hereabouts wanted to see that biddy: we charged grown ups a nickel, kids two cents." In saying "we charged grown ups a nickel" there is a sort of camaraderie and intimacy suggested in these words, since Sook is not included with the "grown ups" but rather is, like Buddy, a child who is making money from selling grown ups a ticket to the museum.

When Christmas morning arrives, Sook and Buddy are both like children, unable to sleep and eager to open presents. Sook mischievously makes noise to wake up everyone in the house, so they can get their Christmas morning started:  

Possibly we doze; but the beginnings of dawn splash us like cold water: we're up, wide-eyed and wandering while we wait for others to waken. Quite deliberately my friend drops a kettle on the kitchen floor.

This is another example of Sook behaving in a childlike way, and Buddy's fond remembrance of her youthful energy and happiness.