The staples we add every year are a small can of diet coke, their own pack of babybel and their own pack of pepperamis (no idea why these are so important but they always seem to be what they get most excited about), something that lights up, a large amount of chocolate and sweets as long as some of it is kinder, there is something lindt and they get a chocolate orange they are happy. I vaguely remember adding a pack of batteries in one when the oldest was little think I was just too tired to function, but he was thrilled and since there is always something they open that we have forgotten batteries for have done it ever since. Actually, think this will be first year that there isn't some kind of stuffed animal included.
Mine like plastic tat lol my 5 year old has some shopkins for her stocking, she fell in love with them at my niece's birthday party and asked for her own after playing with my sisters they are between 8 and 10 if that helps. She also has a love for my little pony blind bags, tsum tsums and those squishy mash'em things. Any tacky plastic jewellery, metal bangles etc for her "treasure box" She's also got a little obsession with craft stuff, notebooks and stickers (hobbycraft, poundland etc are awesome for stuff like that if you have one nearby) and bath stuff think tacky bath confetti, bath sprinkles that change the colour of the water, bath bombs etc.
My 7 year likes blind bags, minifigures, small lego sets, fancy dress accessories like pirate patches, plastic telescopes, silly plastic gold coins, etc from the party sections, whoopie cushions, jenga blocks (they like building with them and are always thrilled to get another set from poundland). Toy cars, plastic figures like soldiers, marbles, playdoh or playdoh cutters always go down well and so does anything "sciencey" magnifying glass etc.
My 14 year old likes food so interesting stuff, stuff I don't buy very often or just stuff he doesn't have to share are always what he seems to like best, pretty sure his favourite stocking filler last year was a bottle of his favourite bbq sauce and a jar of his favourite dip. He's obsessed with pandas so I spend a lot of time looking for strange things online he might like, this year that included a panda stapler, a panda usb stick, a tiny panda flask, a panda torch and wooden panda nesting dolls. Previous hits included panda headphones, panda keyrings, panda phone cases, panda decals or stickers, the sonic the hedgehog that had a suction cup to stick it to the window and glow in the dark headphones. Thinking of adding a gift card or two to his this year, cinema, costa etc since he loves to go out with his friends and probably a back up charging cable since he is always losing his.
For your daughters, how about a book token and a bookmark to allow them to pick a book of their own in addition to the ones you have purchased, a locking diary/notebook since all little girls have secrets they don't want their sisters to know, hair stuff (new brushes, hairbands, clips, maybe sparkly hairspray or hair chalk), purse/wallet, slipper socks, cute gloves/hat, perhaps a new mug or cup, a silly calendar for their room, adult/children's colouring book and pencils, stationary (card factory do little fairy notecards with envelopes that your youngest might still enjoy) but fun post it notes, highlighters, novelty erasers, gel pens or just cute pens still go down well with my niece and my sister, my niece would like nail varnish but my sister has banned her having any more after an incident involving the carpet. Not sure what you have nearby but Claire's, H&M and Primark are usually great for stocking fillers for girls.