what significance does the buster brown conversation have to the developing theme?

There really isn't a conversation. kenny takes off Joey's shoes and discovers they are Buster Brown shoes,

Inside one of her shoes was a kind of worn-down picture of a little white boy with a girl’s hairdo and a smiling dog. In a circle around both of them it said, “Buster Brown... As I drifted back to sleep I wondered what a little white boy would think if he knew he was getting stepped on every day by my sister.

This represents a child's logic on race. Kenny is black and he has seen how whites are put above black people. He finds it ironic that a picture of a white child is being stepped on by his sister.