Is part of the reason they are only doing 1 day because they feel to be dairy they need to give Xmas and boxing day to different parents, and they feel they should spend same time with each, so it kind of ends up having to be only one day unless they are able to arrive with the Xmas day parents a few day earlier, which would be an issue for most people for work.
It might not be that, but in case it is you could drop in that it doesn't matter when over the Xmas period you see them, if they want to come for longer they are welcome to come over new year instead or the week in between, and you'll do a second Xmas then? Less pressure to get to 2 places in 2 days then.
However, it could also be that they have other things they want to do in that holiday time, or just find can't really relax staying with in laws. That's up to them and you cant really do anything about it. If you make them feel too bad, or keep commenting they might avoid coming at all next time!
So i would have one conversation with you son where you made it clear they are welcome to stay for longer if it suits them, and you can be flexible on dates if that makes a difference, but however long it is you are looking forward to seeing them. Then don't mention it again.