Central to understanding this excellent short story is that Kezia and Else are part of the same society, but also come from two different worlds in terms of their class. This is why the Burnells are able to make fun of the Kelvey's and completely exclude them from their games and seeing the Doll's House:
And the only two who stayed outside the ring were the two who were always outside, the little Kelveys. They knew better than to come anywhere near the Burnells.
You might want to also focus on physical descriptions. We are not told much about the description of Kezia, apart from the fact that she is strangely attracted to the light in the Doll's House, but consider how Else is described:
And her little sister, our Else, wore a long white dress, rather like a nightgown, and a pair of little boy's boots. But whatever our Else wore she would have looked strange. She was a tiny wishbone of a child, with cropped hair and enormous solemn eyes - a little white owl. Nobody had ever seen her smile; she scarcely ever spoke.
She is therefore depicted as a very strange, reclusive figure who does not interact much. This is of course in contrast with Kezia, who proves herself to be outgoing, as when she invites the Kelvey sisters in to see the Doll's House.
Of course, what symbolically unites Kezia and Else, although they are so different in every other way, is that they both seem to understand the symbolic significance of the little lamp, and that it represents the warmth of human kindness, that Kezia has just displayed to the Kelveys.