Katherine Rundell was low on money when she won the Waterstones prize for children’s fiction for her novel Rooftoppers nearly four years ago. The £5,000 that came with the award meant the young author could afford tickets to the Amazon to research a new novel, The Explorer – a book that has just landed her the prestigious Costa children’s books prize, won in the past by giants of the genre from Roald Dahl to Philip Pullman.