Riddell (pictured) won the Kate Greenaway Medal for his illustrations for The Sleeper and the Spindle, a retelling of the sleeping beauty fairy tale by author Neil Gaiman.
And a story about conjoined twins by former teacher Crossan earned her the Carnegie Medal – for her "verse novel" One.
Crossan, originally from Dublin, trained as an English and drama teacher at Cambridge University and then moved to the USA where she taught English at a small private school near New York.
She has been shortlisted for the prize twice before, in 2013 and 2015, and has spoken out in favour of libraries and poetry – saying that poetry is being “killed” for children at around Year 8 by testing “leaving no space for joy or performance”. Her book, One, which follows the story of conjoined twins Grace and Tippi who are no longer able to afford homeschooling and must venture into the world, is written in blank verse.
'Librarians are amazing'
Mr Riddell said he was “honoured and humbled” to accept the award and claimed that librarians were “pretty amazing people” because they turn children into readers by teaching them about “the joy of losing yourself in the pages of a good book.”
The 16 books shortlisted for the Carnegie or Kate Greenaway Medal
Sioned Jacques, chair of the CILIP Carnegie and Kate Greenaway medals judging panel, said: “We were blown away by Chris Riddell’s work in The Sleeper and the Spindle; he is surely at the height of his powers.
“Sarah’s book, One, is poignant and thought-provoking, each chapter a poem that is a work of art in its own right.”
The first Amnesty CILIP honours were also awarded for books which celebrate human rights.
The Amnesty CILIP honour from the Carnegie shortlist went to Robin Talley for Lies We Tell Ourselves, a story about the historical battle for equal access to education in the USA. The Amnesty CILIP honour from the Kate Greenaway shortlist went to Ross Collins for There’s A Bear on my Chair, a book which shows how to stand up to bullies in a peaceful and humorous way.