I would second a lot of things on here -
Christmas eve service at a local church
Looking at lights in peoples' windows in streets around
Craft stuff (the works do lots of good bits cheaply) Christmas rocks where you go hiding and finding decorated stones in park? Sparkly pine cones/twigs and cotton wool snow men. A mini Christmas tree from supermarkets to make a snow scene. Nativity sets often in charity shops if that is something you want to include.
Freagle - ask for a fake tree or one in a pot/out of garden - bet there are loads out there. Failing that then head out into nearest woods and find a branch - with decorations in a pot it works.
Food banks have extra donations at Christmas (lots of work places and schools donate for example - that's what we do instead of daft secret santa)
People donate because any of us could be you and your family OP. One day when you have come through to better times money wise, you will be the ones donating - think of it that way.
DVD player - Freagle or ask at charity shops.
Buy a few bits every week as you see on offer/in pound shops - stuffing, cranberry sauce etc Hot choc and marshmallows. Little pud.
Instead of films you can play games and read Christmas stories - millions in charity shops in Nov/Dec.
One little treat we have is to buy a nice Christmas smelling bubble bath. Waitrose do a clementine and ginger one for £2 and we all enjoy a festive bath.
Happy if you want to inbox me. I will post you one or two bits in Christmas wrapping if you want. Its exciting getting thing by post at any age.
Concentrate on the fact that you are a loving family with two lovely children. That's a wonderful gift. The rest of the stuff you make up as you go along and it doesn't have to cost a bomb to be special. Good luck and come and tell us about your Christmas later on. Would love to hear how it goes xX