BookBrunch - Do you have to 'transcend' a genre to win a prize?

Perhaps there is a case, for the sake of neatness, in restricting the Man Booker to adult novels. Only two children's authors have won the Costa - previously Whitbread - Book of the Year since the current format was introduced in 1984. But would Philip Pullman's The Amber Spyglass or Frances Hardinge's The Lie Tree not have been worthy Booker winners?