We had this one asked after on the Science Fiction and Fantasy Stack Exchange, Children find hole in their bedroom wall which changes objects. It is Nicholas Fisk's "Swap Shop" from his collection Sweets from a stranger.
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)