OK - some suggestions to help with immediate Christmas issues:
Talk to your DCs. Acknowledge that it is exciting and lovely to get presents (all kids love presents and yours are still quite young) and they will be getting some (though not everything they want) but Christmas is not all about buying things and is about being family together and about giving too, so as well as presents you will be doing some extra things.
a) One poster talked about carol singing to raise money for charity. How about getting them to choose a charity (you could look up a few on t'internet together, or do something in the news like giving to flood victims in Philippines) Maybe you and your DCs could cook or ice some Christmas biscuits #( or failing that get a tin of Quality Street or similar). Then go round and sing carols to neighbours and offer a biscuit, wish them Merry Christmas and ask for donation to chosen charity. Or just do the biscuits and singing as a "gift" to your neighbours.
b) To emphasise the "it's not all about money/getting" thing but still have a positive spin, maybe you could get them to come up with things they'd like to do as a family over Christmas that cost little or nothing (and maybe don't involve electronics/ds/computer). Things like playing a board game, watching a family movie together with sweets/popcorn, maybe even playing together with Lego or other toys they already have- depends what your dc's would like. They get to choose one thing each but so do you and DH.
c) Do a pre-Christmas sort out of old toys they no longer want (to make room for new), stash in garage or somewhere to be taken to charity shop in New Year. To make it a family thing, maybe you and DH could sort out some stuff too.
d) Talk now - before it happens- about all the extra things that have to be done so that everyone can enjoy Christmas. Maybe talk about how the family is a team and that this year it will be a team effort. Decide now how they will help for the next few days and on Christmas Day itself and get their agreement beforehand. If they say "no I'm not doing that", then ask what they are going to do instead. They have to come up with some things that are reasonable for their age and ability. (Could be tidying room, dusting for pre-Christmas and on the day - collecting up wrapping paper, collecting dishes, emptying dishwasher, making cups of tea/coffee (for eldest one)). Let them know when and how they will need to do the job(s). I think children respond much better when they know in advance what is required and when they have to do it. Include the jobs you and DH will be doing too and roughly when you will do those so that they get a feel for what you will have to do over the next few days and feel that you are all in it together.