I have DD’s so not much help with your boys I’m afraid, however budget ideas for your girl...
How about a treat night hamper, bath bomb, book/DVD new PJ’s hot chocolate with nice mug (definitely could do this around the £12-£15 marker if you stick to Poundland/B&M/EBay.
Bits for her room, Aldi had nice bedding in a bag for £9 geometric print, pink rose gold etc, B&M are also great for the sparkle rose gold accessories, arty quotes, tea tree light holders, they also have lots of girls fairy lights.
Wireless mouse/headphones on Amazon, managed to get my daughter a rose gold one on kne of their Daily deals for around £10 and keeping a lookout for the headphones...
Primark for Harry Potter things, cheap as chips but very current, house mugs, scarfs, badges...
Books, there are lots of places that have deals on the book people, The works etc however I never look past amazon and if you are not looking for anything specific always find a bargain with free delivery.
Superdrug for makeup girly bits, always seems to have things discounted and has 2 for 3 on everything at the moment, lots of girly nail varnish’s lip gloss, sparkles (my girls aren’t allowed anything too grown up) I spent around £15 the other day and got lots!! They also have loads of curling wands/tongs reduced (around £15-20)
Craft/science sets from B&M around the £5-7 marker, always find them good sack bulkers and last the entire year as we pull them out on a rainy Sunday afternoon
Squishies on EBay all 3 of mine are obsessed and they are only a couple of ££ on EBay with free delivery with lots of options, mine arrived really quick too!
For a joint present you could go down the the Alexa route the 2nd generation was reduced to £19.00 for one of Amazons daily deals, keep an eye out as it might come down again...
Movie hamper, couple of DVD’s snacks, drinks, fluffy blanket etc?
Anyway hope that helps. 