“Lawyers, I suppose, were children once.” – Charles Lamb
First, it's from an essay by Charles Lamb, an English writer in the late-18th and early-19th century. (Check out the original source here.) And it sets us up to read the story in a few different ways:
There's a time-honored tradition of making fun of lawyers as not quite human (even Shakespeare got in on that fun). By reminding us that lawyers were children, too, Lamb (and Lee) is telling us that lawyers—like Boo, and like Tom Robinson—are human, just like the rest of us.
Lawyers can seem the opposite of children: while kids are innocent and say just what they feel (no matter how embarrassing), lawyers plot and scheme and say whatever they need to in order to win their case (or so the stereotype goes). Linking lawyers to children suggests these two opposites perhaps aren't so different after all.
It also sets up a link between Atticus (the lawyer) and Scout (the child). We don't know what Scout grows up to be, but, considering the way Miss Caroline ribs her at the missionary tea, we think it's a possibility that she becomes a lawyer. Maybe To Kill a Mockingbirdshows us how a child witnessing injustice can grow up to help change it—as a lawyer.
Nice job, Lee. That's a lot of work for six words.