No, A.S. Byatt has not written a children's book, she has written The Children's Book. I first heard about it at the Guardian books blog, where I get all my British book news ("The stories children's books tell about the world they're written in," 4/27/09), then tracked down the flap copy:

"Olive Wellwood is a famous writer, interviewed with her children gathered at her knee. For each of them she writes a separate private book, bound in different colours and placed on a shelf. In their rambling house near Romney Marsh they play in a story-book world - but their lives, and those of their rich cousins, children of a city stockbroker, and their friends, the son and daughter of a curator at the new Victoria and Albert Museum, are already inscribed with mystery. Each family carries its own secrets."

There's more, notably a German puppeteer, but that was enough for me.  The Children's Book is available from amazon.co.uk on May 7, and I'm tempted to order it from them now rather than wait for the US edition in October.  Maybe I could pick up Kazuo Ishiguro's Nocturnes: Five Stories of Music and Nightfall at the same time, just to make it worth it?