So sorry to haer of your loss , it's difficult losing a loved one at any time but seems so much worse at Christmas. As hard as it's going to be, I really agree with the other posts - you need to try to celebrate Christmas as best you can, for the children's sake. Your mum wouldn't want you and your children to have a miserable Christmas. Be as strong as you can, do as much as you can for the children, and though there will be times that you need to take yourself away for a few minutes when it all gets a bit much, overall I'm sure you can make it a happy time for the kids.
Much love xxxx