Lee's portrayal of the children is wonderful - accurate, funny, and charming. It's also a great way to tell a story. Told with hindsight Scout is able to fill in details that she otherwise wouldn't have known at the time, but it also enables the author to view the events through the prism of a child's view of the world. The novel's central theme is prejudice whether it's prejudice on a child's scale - that of the scary old woman, the odd neighbour, the "other", or prejudice on an adult scale - focusing principally on race.
It's a very serious novel, with a tragic turn of events at its heart, but contrasting with this it's also one of the best portrayals of childhood ever. It's also a wonderful portrayal of the Deep South with the little town that Scout and Jem, her brother live in, almost being an extra character in the novel. The eccentricities, charm, good manners and prejudices of the South are atmospherically portrayed, you can almost hear the mockingbirds singing. Many people have regretted that Harper Lee never wrote another novel, perhaps she just realised that nothing would ever be able to compare with To kill a mockingbird.