The winner in the children's category is:
The Bower Bird, by Ann Kelley
As I seem to have missed the shortlist announcement*, these are the other books that were in the running:
Other category winners at the Costa website.
*It's perfectly possible that the holiday season has caused a lapse in my memory. If so, I humbly apologize for repeating myself. All of the fudge drained my brain power.
**Which is being published in the States as a grown-up book. Well, I assume it is -- Amazon has listed the publisher as Viking Adult, so I admit I'm taking a big leap there.