Saffy's Angel by Hilary McKay beat the three other shortlisted books to win the title.
Hilary told CBBC Newsround's website that she was thrilled just to be shortlisted.
She said: "I certainly didn't expect to win.
Now Hilary and the winners of the other four categories in the Whitbread Awards will have to wait until 28 January to find out who the overall winner is.
Last year Philip Pullman won the children's category with his novel Amber Spyglass and went on to win the Whitbread Book of the Year award.
But he was the first ever children's writer to do this (even JK didn't manage it), so we'll have to keep our fingers crossed for Hilary McKay and Saffy's Angel.