I'm so sorry to hear your awful news OP. I am right here with you. I've not much of use to offer, although I have experience of losing my mum not too long ago and this is what I can help with;
Allow yourself as much time as you need to come to terms with this. It's a huge shock and you need to be kind with yourself. Sit with each emotion as it comes, feel it, process it, know that it will pass and another feeling will come. Not all of them will hurt. I promise you.
Use the macmillan nurses, they're wonderful and are there for you and the children, as well as DH.
Take pictures, make scrapbooks, photo albums..
Take it step by step, hour by hour, day by day.
Plan what you can together but don't feel that you need to fill every day - just being together as a family is enough
There are no words that anyone can say to make it better, but I hope we can offer some comfort by being here for you.
My thoughts are with you, your DH and your family. 