My dds are similar ages to yours (15 and 11) I have got quite a few gifts from vinted. (Set up alerts on items you are looking for)
I got book sets they wanted to read (dd1 various manga, dd2 heartstopper novels). Figures they collect, switch/playstation game each, board game each all at a fraction of the RRP price.
Large squishmallow for their rooms - they love to lean on them when gaming or use as pillows. (The entertainer had a fantastic deal a while back on these!)
I used temu and shein for stocking fillers - pin badges, tshirt & hoodie each (demon slayer for eldest, Taylor swift for the youngest!), beads & jewellery making for youngest, marker pens for my eldest. Fun book marks, kawaii style stationery for school, keyring for their keys, themed dobble (hello kitty & demon slayer), earrings/ stretchy choker necklaces, fluffy socks.
I got some body sprays from superdrug deals, some beauty face masks from primark and fake nails (for youngest) and nail varnish and stickers for eldest (stickers from shein). Lip balms and hand cream each too.
I used to get a lush bath bomb but will give them a voucher each as they go half price after Christmas so they can get double the amount plus they love it in there! Will probably get them each a Costa voucher too as they love a latte or hot chocolate when out with friends!
They will get a box of pop tarts each, individual boba kit, bag of percy pigs, chocolate orange, toffifee, Toblerone and a giant chocolate coin each in their stockings.
Join the bargain thread, loads of deals and ideas there!
If they haven't got an oodie type hoodie that's a must! mine live in theirs! I got some actual oodie seconds from ebay as shared on the bargain thread (2 for £26), they've come and are fantastic quality, so heavy and warm, just had the tags cut off and will need a freshen up as slightly chemical smell. I will wash when they are at school and hang in my room. Or primark had loads of theirs reduced this week if you can get instore.