@nonotchristmas
Firstly ignore the self righteous idiots of this thread with the 'poor children' comments. The fact that they gleefully stick the knife in says more about them than you.
Now, its bloody tough with two that age, no matter what anyone says. So recognising that doesn't make you weak or a bad mum, it makes you a fallible human just like the rest of us.
I remember a Christmas when my mum didn't have long left and was in the hospice, I had two your age and both DH and I had the flu. I was literally on my knees mentally and physically with a big dollop of feeling guilty that the kids wouldn't have a good Christmas. So I just let go and thought just getting through the day was enough. That next Christmas they would be older and it would get better, and it did. So take the mental pressure off yourself. It gets easier with them as they get older so just keep in mind that they won't always be like this.
All I did that Christmas was put back to back films/kids TV on, leave making stuff out and toys to play with, opened snacks and stayed in PJs. I just surrendered. It was an awful Christmas for the adults but the kids didn't notice and they survived.
Obviously your situation is different and a useless DH has to addressed at some point or it will cause long term damage. But for this Christmas, you do have some breaks with family around. Don't be afraid of being vocal in asking for help. It's so isolating trying to do everything yourself. Ask your ILs to entertain the kids, put your DH on the spot to help. Push back, don't take it all on yourself.
Activities to wear them out is the key, walking is a great idea and lots of fresh air. Can your eldest kick a ball? Give DH a ball and send him to thr Park with the eldest.
If you have to go to a soft play then just grit your teeth and use it for all its worth. Anything to keep your sanity.
Try to have a little plan for each day, it will give you routine and shape the day with tasks.
Good luck and keep your chin up. It does get easier.