OK, so the kids can make snow flakes out of circles of white paper. Fold them in half 3 or 4 times and then cut into them. When you open it out its like a doilie and they can stuck them on the windows.
You can buy polystyrene balls in Hobby craft and little pins & sequins for you, to make baubles.
You could get the kids to stick 2 old cd's back to back and decorate with glitter glue. Stick a piece of ribbon on the top and hang.
Toilet roll tubes and ping pong balls make great snow men covered with cotton wool.
Fun foam cristmas cards. Hobby craft do loads of that and will have loads of christmas shapes in now.
I'll keep thinking!
Felt - snowman shapes, robin, christmas pudding, tree shapes, stars, hearts etc. The kids can help you sew them and you can use glitter pens to decorate them. If you go to www.flickr.com and search for felt christmas decorations you will find lots of pics of ones people have made for inspiration.