At the end of the novel, when Atticus is reading to Scout, she comments on the character in the book by saying: "When they finally saw him, why he hadn't done any of those things. Atticus, he was real nice." To which Atticus responds, "Most people are, once you see them."
Human innocence is drowned out by prejudice. Most of the characters in this story are "innocent" of the claims being made against them - Mrs. Dubose, Tom, Boo Radley, Atticus (at times), even Scout. However, the prejudice of all the characters, from Scout to Jem to Miss Caroline to the town, causes them to find guilt in others around them.
Courage is looking beyond your own point of view. A common theme in this story is that a person must "walk around in another's shoes." The most courageous and heroic characters are those that are able to break the bonds of their perspective and see the world through another person's eyes.
You could try true courage is doing the right thing even when you know you can't win. This is obviously what Atticus is doing. He could have turned the Tom Robinson case down, but as he tells Jem and Scout, had he done that, he could never tell them to mind him again because he would lose all respect for himself. Real courage is standing up for what you believe is right, even when it is unpopular and even dangerous.
For innocence you could try something like - Tom's death ends a period of innocence for all of Maycomb. It signals the end of the way things used to be (the old southern traditions and racist environment). When Atticus is able to get the jury to deliberate for so long, it is a victory in itself. The fact that they were unable to simply apply old southern justice and lynch Tom is another sign that things are changing.
That brings us to your final theme - childhood. For this you might want to connect it to the them of innocence too. But if you don't, you could focus on something like - often it takes the goodness inherent in a child to show people the error of their ways. In this I'm thinking about that scene ch 15 where Scout ends up standing up to the lynch mob. Scout recognizes Mr. Cunningham and appeals to what little good is left in him. Because of this, Mr. Cunningham makes the mob leave.