Start checking out charity shops for ideas and bits. I got a plush toy my 9yo was desperate for and I couldn’t find anywhere for £2 the other day just by taking shelter from the rain in a charity shop I’d not been in before. The same 9yo loves her makeup, I got her a perfect pink make up case a couple of years ago for £4 from a charity shop. Last year we upgraded it and I got her one from home bargains. I usually buy bits of makeup there for all 3 of my DDs, it’s cheap and I don’t mind them trashing it. My 9yo is the only one who actually cares about makeup though so she also gets nicer bits, for these I pick bits up through out the year when I see them on sale. We have a catalog outlet near us so I’m lucky I can get bits in there regularly. Last year I managed to fill the new case with loads of stuff for her really cheaply.
One of my DDs also got a squishy mellow from a charity shop recently, looked brand new.
To make things look bigger and fun on the day you could get creative with boxing, a box in a box or shredded/tissue paper in a giant box and they could hunt for their presents (ALDIs big crisp boxes are great)
You can buy your own build a bear type sets, I’ve seen them in Aldi, maybe make some keepsakes to put in side them, a heart, secret message to Santa etc.. you could buy an extra bag of cushion filler to fill them out more. My DD uses it to make her own squishy toys, she makes the outer layer out of paper and cello tape then stuffs the filler inside to make her own toy. She’s also teaching herself how to crochet as she wants to make little crochet dolls and animals. Would you older dd be able to do something like this?
Not everyone is a fan but I like shein and aliexpress for stocking fillers and novelty bits.
Also keep an eye out for the Hobbycraft sale, I think it was September last year, they had loads of great arts and crafts stuff on sale, I bought a load of art stuff really cheap last year.