I agree with Claires, their sales are usually quite good and then have additional 10-15% off codes on top. I picked up lots of instax camera accessories including phone case, photo frames, keyring from the sale around £1 each, lipgloss, hairbands and scrunchies, 9 pack of earrings all around £1.
Christmas themed sweets - cadbury puds, aero snowballs, reeses christmas trees, choc coins etc
Will also do some blind bag bits, preeteen is into among us so will get those, but roblox, lego, minecraft and pokemon cards for younger ones too.
More expensive items you could include instax film, gift cards either for places like nandos, wagamamas or ingame currency like roblox etc.
If they are still into smiggle at that age they do some christmassy scented pencils.