Make the most of the run up to Christmas. Walk around your local area counting Christmas trees in people's houses, ask on your local FB page where the most decorated houses are and go visit. Check out local garden centres which have eg singing reindeer and their trees and decorations set up to make just a walk through a magical experience.
As PP said, make paper chains. You could go truly traditional and cut the strips out of magazines/newspapers/wrapping paper.
Make Christmas cards for relatives.
The Christmas rituals - carrots for Rudolph, mince pie for Santa... will bring their own magic. In our house, Santa used to leave a box of coco pops, wrapped and with a 'thank you' card from Santa where the carrots etc had been. They didn't have sugary cereal usually, which made Santa seem 'real' as the stuffy old parents would never have allowed it !
Make them a little breakfast gift bag with a carton of juice/milk shake and a breakfast bar & satsuma or banana so they can have a breakfast picnic on your bed whilst you snooze a bit.
Find a film you'd all enjoy and snuggle up and watch on Christmas Day with a big bag of popcorn - take the emphasis off the presents. In fact you could make up a 'movie hamper' and turn the popcorn and maybe some drinks/sweets/crisps into an extra present.
Re presents - it really isn't about the money ( although I know many on here disagree judging by other threads)
Think about ways of making little gifts special
Eg Make each child a 'bran tub' so they have to dig through the 'bran' to pull out the little presents (if you or a friend has a paper shredder, making the 'bran' is easy and you can ask your local shops for empty boxes)
Do a Christmas pass the parcel with a chocolate coin in between each layer.
Put some children's Christmas tunes on and have a disco.
Make the most of small things - see pics for ideas for decorating chocolate. Aldi Choceur chocolate is fab (in boxes, easier to wrap) .
Definitely don't worry about buying second hand from charity shops or local Facebook groups.
If you still have a pound shop, trawl for bargains.
Wrap up things they are going to need anyway - clothes, bubble bath etc
The unwrapping is as magical as the actual gift in my experience.
You'll get tons more ideas off this thread. Stop stressing about the money and make the most of the Christmas magic. I promise you they will remember their Christmases fondly x