My book, Hunger Winter, got started in a surprising way. My son, who was then in 7th grade, came home with a story he'd dashed off during study hall. I thought his story was pretty good and since I was already a published author, I asked him if the two of us could write it together, just for fun. I didn't intend to publish it.
I should have known that life was snickering at me when I said I wouldn't publish it.. Eight years later it's published with some great endorsements. You live and you learn.