Last time we spent Christmas with PILs it did not go well.
I had 16 people in my small terraced house including my SIL's PILs, as her DH is an only child....I was trying to be nice!
Anyway the actual cooking was fine and not too stressful, but naturally, I needed to spend a fair amount of time popping in and out of the kitchen.
MIL took charge of DCs and their presents (were 1 & 3 at the time) and I missed seeing them open their main presents. I did ask her to wait for me (only a few minutes) but "it wasn't fair to make them wait". They would have been quite happy with some paper for a bit.
In my family presents are opened one at a time and we all enjoy seeing what others have received etc, DH is used to more of a scrum and I'm not sure which is right, but it can't be unreasonable to want to see my DCs open their presents surely?