I agree budget £30 each and £10 for cheap stocking fillers.
Stockings I'd go somewhere like home bargains. Choc coins, chupa lolly, flump sticks, etc. Bubble bath/felt pens/ stickers/ 99p bath bomb/ cheap torch/ etc.
For their £30 each (stick it in named envelopes) I'd go places like poundshop, cex, primark, the works. They all do cheap gifts.
Things like headphones, slime, playing cards, dominoes, books, dvds, T-shirt each of something they're into, etc. My son is 13 and likes the men's primark licencsed stuff eg Mario socks, pants, backpack, tshirts, and so on.
I'd focus on family time having quality time with them eg board games together, movie and popcorn, geocache walk, that kind of thing.