The Minpins by Roald Dahl, Patrick Benson

Little Billy strays into the forest, where he meets the Minpins – tiny people who live within the trees. The Minpins tell Billy about The Gruncher, who preys on them. So Billy embarks on a mission to rid the Minpins of their foe once and for all, and sets off – on the back of a swan – to confront The Gruncher.