Yesterday my son Ben, age 6, brought me Dan Barker's "Maybe Right, Maybe Wrong" and told me he had read it. I hadn't asked him to. He just found it in his bookcase and started reading, and apparently finished the thing on his own.
I was a bit skeptical since I've never yet known him to read anything that long, so I asked him some questions about it. He remembered the part where the girl had to decide whether to put her cat to sleep. I asked him why she decided to do it. He thought for a few seconds and said "Because of her principles. But it's not like the principal in school."
Then he wanted to read the book to me, and I noticed that he does voices like I do when I read. It's subtle, but in a cartoon scene where a kid and an adult are talking, the kid definitely has a higher voice.
That book, by the way, is signed by Dan Barker: "For Russell and Ben, Freethinking Friends."