I somehow missed this short story from Roald Dahl during my childhood. It is a fun short read with the usual Dahl silliness: Fabulous made up words, tiny creatures, an ill-defined monster that is somehow more monstrous because of the lack of clarity.
I was a bit worried at the start that it was going to be weirdly religious with the mysterious "forest of Sin" that Billy was forbidden to enter and no-one ever returned from

Beware, beware the Forest of Sin,
None come out, but many go in

and the mother's constant questioning "Are you being GOOD Billy?" but after the introduction the religious overtones disappeared and this became just a very Roald Dahl story.