Any answer to this question should not provide specific information given the very nature of meaning within symbols. Look to items particularly associated with each character. A symbol of Atticus may be his glasses, his favourite chair, even the rifle. Calpurnia may be symbolized by her association to the family through food and through her church. There are, in other words, as wide variety of symbols which may be associated with each character, but it is their interaction with these items, and how these items play out in light of the rest of the story, and in light of how these characters are perceived by other characters. Symbols only have meaning that we, the reader, attatch to them, even when that meaning is an assumed one "meant" by the author.
I wouldn't think glasses is a symbol in this story, but possibly the dog. The tree could be symbolized for the children also.