Daughter and son-in-law not happy with each other

I would have a word with your daughter and get her to agree to a truce with her husband just for the day. No sniping and no bad feelings.. If they can't agree to do that for the sake of their son and for you I would cancel hosting them..

Can’t help, sorry, but in a similar position so sympathise.
It’s just a day. If weather permits go out for a walk, the fresh air will tire the toddler and they may even leave early….

I'm tempted to agree with pascal but what about the child?

Having a word with your D is a good idea and I hope the day isn't spoiled for you but at least if they are with you, your GC has a better chance of a happy day.

Maybe you could talk to them both and point out that their constant arguing is having a detrimental affect on their child.

Sympathize with you but they do need to be told about the detrimental effect on the poor child as Christmas is a magical time for young children isn't it? Are you brave enough to confront them tho,?
Difficult situation.

Does your SIL have to be there? Can't he go to his parents instead. You deserve a nice day.

I would only address this if something happens in the moment. Tread carefully here.

Selfish pair. The child will be picking up on it but they know that. I wouldn't say anything to parents, your comments will not be welcome. I would give my attention to the little boy, Christmas is for children and he will be excited so give him time. Arrange a couple of little suprises for for him once they start.

It's just one day.

This is obviously going to spoil the day for everyone so I would do as Freshair suggests and tell your son-in-law to spend the day with his own family.

I agree with Allsorts poor child having parents sniping! I would have a word with your daughter , sounds as though they are both as bad as each other, really must put their little child first I really hope they sort out their differences of opinions not just for Christmas but for the future and certainly not fair on the little one, I hope you have a calm Christmas Dotty

I think you need to be careful not to be seen taking sides. Telling sil to go to his parents is a definite no no for me. You really need a little chat with both parents to say let’s put aside differences and be nice to each other over Christmas and make sure gs has a lovely day with good memories.

I agree with Silverlining48 about a walk. Fresh air and exercise rarely cause rows. Otherwise can you suggest they go out together while you look after the little boy? Maybe a couple of hours on their own might help them relax.

You don't say if you are cooking Christmas day, if so this could complicate things. Do you have a partner?
I was thinking you could take control of the little one and purchase some suitable toys which you both could play with while your SIL and daughter go out for a couple of hours and they might try and come to a compromise for the day.
Having time as a couple if both are working can be hard work on a relationship this could give them a little breathing space to talk.
There are lots of alternatives to this, your other half could cook or your daughter, you could just prepare a buffet to make it easier than a full Christmas lunch. Most of this could be prepared the day before.