Our stockings are regular sized thankfully so we don't need to spend too much filling them. We also only do treats in stockings, nothing practical like toothbrushes or pants.
I've only been to M&S so far and picked up:
Percy Pig Choc Corn Tablet
Percy Pig Mini Sty of Wonder
Colin The Caterpillar Candy Cane Jelly Sweets
Colin the Caterpillar Hot Chocolate Stirrer
I also saw the chocolate topple tower, the percy snoses, chocolate coins and winter wonderbar which may possibly also make their way in.
My teen DD will be getting a few lush products, gift cards she can use when she goes out with her friends, festive hair scrunchies/earrings/alic bands etc, maybe some festive editions of nice nail polishes. I'm going to just pick things up as I see things she would like I think.
The Older to pre teen, will be mostly sweets, scrunchies etc as above, minecraft/roblox/fall guys festive versions of anything like that, blind bags, asmr type toys, maybe either robux or minecoins for their games and smiggle festive scented pencils look good.
And I hear a mini pringles tub is always a hit with teens so may do that too!