It's the first children's book to win in more than a decade, but the judges said "we all loved this dark, sprawling, fiercely clever novel that blends history and fantasy in a way that will grip readers of all ages".
Edited to add: a quote from this article by Philip Womack -
"The Lie Tree is a substantial text that is complex and intelligent: a lustrous, delicious romp about evolution and feminism, which contains a Hamlet-esque revenge plot. Crucially, [...] it comes from a place of wonder. Regardless of the age of the reader, our whole literary system is predicated on wonder, a fact that is often neglected [...]"