You could re-model your existing Christmas decorations. I give old garlands and wreaths a new look by simply changing the ornamentation on them. Look in pound shops for bargain baubles, and also Wilkinson stores.
Gather pine cones to spray gold or silver, they can be tied to a string or ribbon for a garland or attach loops to hang on the tree.
The younger children could do potato print pictures, or use them to make cards for relatives, e.g. a Christmas tree shape can be embellished with coloured sequins as baubles.
Cut out 'snowflakes' from white paper and stick to windows.
Cut star shapes from cereal packets and cover with kitchen foil. Hang from ceiling or stairways with thread where they will catch the light as they revolve.
Make paper chains from strips of foil type wrapping paper and stick loops together with sticky tape. They stay stuck together well and sequins can be added for extra glitz. You can get the older ones to cut up the strips in advance and all join in!
Think about including more functional items for the children's presents, like clothes, underwear, socks, nightwear, toiletries etc and other things they are likely to need.
If you like crackers, look at making your own. Hobbycraft do a pack of six for £2.99 I think, and you assemble them yourself and insert your own choice of gift. This seems more expensive, but you can personalise them and the gift can be something that is actually wanted. You can put things like small jewellery, eyeshadow, lipgloss, pen, keyring, hotwheels car, tiny doll/teddy, mini shower gel/handcream/soap, handbag mirror, hair slides/scrunchies, superbouncy ball, pocket money toys, novelty pencil sharpeners etc. I usually include a sweet in each as well. It extends the present opening and seems very luxurious to have personalised crackers.
Hope there are some ideas here that you can use and that your family will enjoy.