Scare Yourself to Sleep by Rose Impey, 1988.
A girl and her cousin are camping out in the backyard when they decide to play a game they made up themselves called Scare Yourself to Sleep. They take turns inventing and telling stories about monsters lurking out in the darkness. The winner of the game is the one who successfully scares the other.
They talk about the Garbage Goblins, who live in trash cans. The Flying Cat is a creature that is half-cat, half-moth. The Tree Creeper lives in trees and looks like a branch, just waiting to fall down on someone. But, the best of all is the Invisible Man!
However, their stories are interrupted by the girl’s brother, Simon, who is trying to scare them . . . at least, they think it’s probably Simon. The monsters are made up, aren’t they?
I first read this when I was about seven years old, and I’ve loved it ever since! The pictures show not only what’s happening in the girls’ imaginations but hint at what’s happening in real life, just outside their tent as Simon acts out the things in their stories. It might be a little too scary for very young children, but kids about seven and up will like it (provided that they also like creepy stories). The illustrations are vivid and detailed, and readers may be wondering up until the very end how much is really imagined. All the books in the Creepies Series are about the power of imagination and how it can be fun to be a little bit scared.