You are very much not being unreasonable.
BUT maybe you have to try to power on for your other kids?
I don't like Christmas because of links to upsetting things that happened in my family. Not as bad as yours obviously, but distressing when I was a child.
You had the worst thing imaginable happen, and your younger children were involved too as you say. I'm absolutely not minimising that.
But your younger children are still children and it's not healthy for them (or you) to put their (or your) lives on hold. As we get older Christmas is a little bittersweet as we miss people who've left us. It's a shame it's happened so early for them and in such a horrid way. And absolutely devastating for a parent.
Things can't go back to how they were. There will always be a huge gap. You all loved your son and he will be missed forever.
I think you need to try to find new traditions as well as remembering him on the day. Can you do something different? Nobody has any cash, but can you get away so you don't have to be at home? Or can you be in a bigger group for part of the day so you are distracted for at least a few hours?
I hope I haven't come across as heartless. You must remember him on the day. But if you can have a little happiness in being together and even doing something fun that wouldn't be disrespectful to him. And as a parent sometimes we have to fake it til we make it and put on a brave face.
I wish I could give you a hug.