Five Children and It by E Nesbit

Be careful what you wish for -- you may get it! That's what happens to five children when they decide to dig a hole through the Earth to see whether people on the other side walk upside down. They don't get very far, though, before they uncover an ancient sand fairy. At least that's what the youngsters call him, even though his correct name is Psammead (pronounced Sammyadd). And what a bizarre creature he is, with bat's ears, a tubby body, furry arms and legs, and eyes that move in and out like telescopes!
Obliged to grant the children their desires (because that's what sand fairies do), this oddity from another time and place warns of a catch: wishes come undone at sunset. And if they're not planned carefully, there could be some very serious problems.
One of Edith Nesbit's best-loved tales of enchantment, Five Children and It will delight today's young readers as much as it did those of generations ago.