Children find hole in their bedroom wall which changes objects

Question

I'm trying to remember the name of a short story I read about 25 years ago or so. I'd also like to know of any collections it appeared in.

The details I recall concern a pair of young children ( brothers perhaps) who discover a strange hole in their bedroom wall.

Somehow they discover that if they put something in it disappears and comes back changed slightly. The changes usually involve some kind of rotating/twisting jewel or bead being incorporated in some way.

They pass a few inanimate objects though, and then they decide to put the family cat through. It comes back with these jewels for eyes and acts very strangely.

I cannot remember how the story ends, it may end with one of the children going through the hole, but I cannot be sure.

Answer 1

Nicholas Fisk - Swap Shop (from his collection "Sweets from a stranger").

Terrified me as a child. The ending to this story in particular. From this review:

It begins with the wind whistling through the wall. It’s not the noise itself that’s the problem, says Jo, it’s the feeling behind it. “It’s almost as if someone or something is saying things in the wall…” (103). Her brother Bogey (nee Alec) teases her about her fears, but together they reveal a hole in the wall, a hole that is “all wrong”, that is sometimes a normal hole, and sometimes “seemed to shift — to move, to swell and contract, almost to breathe” (105). Neither sibling is brave enough to venture into the hole, but Bogey throws in a old, cracked, china mug. It disappears. Two minutes later, a glass tumbler appears: thick, whole, beautiful green glass.

Further swaps ensue. They put hot chocolate in the glass, and get back a golden liquid that tastes of every fruit and none. They put in salted peanuts, and get back unsalted. A needle and thread, and get back two pieces of fabric joined by a small button containing a golden worm, that glows “like the filament of a torch bulb when the battery’s almost flat” (109), and slowly rotates. Other devices come back with other worm-buttons on. Bogey gets excited: this could be his fortune! These miraculous worm-buttons, which seem to be able to join and clean and power and much else. He tries to establish direct communications with the whatevers on the other side of the hole: his notes and photographs come back unchanged. He does the inevitable. This is what Jo finds in the morning:

Motionless, but for the fluttering of the petal-like eyelids. Glimmering white, smooth, flawless, hairless. Him. Not him. His head seemed larger. Too large. His scarred lip was still healing — as she watched, the last of the scar faded and vanished leaving only rose and white perfection.

He groaned and rolled from side to side; then completely over. She felt the burn of vomit in her throat when her eyes were trapped by the sight of the crystal buttons in which turned little golden worms, in his neck, his brow, his belly, his chest. His eyelids fluttered again. They opened: then she saw the spiralling golden worms in his eyes. (121)

Answer 2

It has been a while since I have read it, but I am reminded of Mimsy Were the Borogoves by Lewis Padgett, about a box of educational toys from the future.