Mr. Underwood referenced the "senseless killing of songbirds". The mockingbird is known for having a very pleasing songbird sound.
He wrote this article in reference to the shooting death of Tom Robinson. This compares Tom to the mockingbird. Tom had not done anything, just like mockingbirds don't do anything but sing for us.
Mr. Underwood was generally understood to be racist. This article positions him in defense of a Negro which demonstrates that steps are being made slowly but surely in the community surrounding Maycomb.
This strikes a cord with the kids because previously they thought of birds as unimportant. The principle is clear to them now that they see it played out in humanity.