Mockingbirds within the novel, are primarily symbols of innocence. It also symbolizes goodness and beauty. When Miss Maudie first explains to Scout the meaning of Atticus' statement -- that it's a sin to kill a mockingbird -- she agrees with him.
Jem and Scout particularly symbolize innocence, goodness and beauty too. Although they are a bit more rambunctious than some children, their childlike questions and responses to the good and bad around them and their innate willingness to foster goodness in their actions make them positive symbols for the future of Maycomb.