I am really, really sorry that this has happened to you. Its a pain like no other. I lost my little boy an hour after he was born and the grief was unreal.
When you are a mum, with another child who needs you and doesn't really get what has happened you really don't have much choice but to try and 'pretend' you are okay when you are with them, then when alone let it all out and cry, then pull your big girl pants up and go again. It shouldn't be like this, but that is the reality. And I agree, its bloody hard when you heart aches like never before.
Are you close to your parents? Do you have siblings/friends who can take some load off and do some things with your other child while you take time to do the practical stuff and allow yourself the space to grieve? My Mum and Dad were my rock when my little boy died and I honestly don't know how I coped without them, even though they were heavily grieving the loss of their grandson too. But we were all in it together, and we all supported one another and did our best.
Like a previous poster said, you don't need to create the best Christmas ever, just as good a one as you can muster up for your child. They will not really remember it much and won't care too much as it will still be a magical day.
I also want to say that I totally understand the fear of losing your other child. The pain of losing a child is horrific and you suddenly become so scared of something happening to the other. It is very normal for you to be feeling this way. That fear will fade, your grief will lessen over time and life will get back on track but it will take time.
Sending hugs and I hope that you manage to give your little boy a lovely send off. It will be the hardest thing you will do, but you will get through this, I promise and there is a life at the other side of this grief.
xx