Sorry, I was typing that earlier on the phone while waiting for my train.
A set of things "for the trip" sounds good. New Pjs - which can be festive as they will get well used for the week+, or not - are a good idea. And maybe a sachet of hot chocolate - to be able to make en route with a flask of hot water when driving or cup of hot water on ferry/plane - they may well be able to do it anyway, but having a sachet yourselves means you only need hot water if they don't have it. (And some mini marshmallows for on top).
When DD was small (I think she was about 4 when we first did longhaul), we used to wrap up very small presents (pocket money toys) and let her open 1 an hour on the flights. THings like a small lego set, mini pack of modelling clay, a favourite comic and (you know those plastic sheets in a cardboard sleeve that you write with a plastic "pen", lines appear, but when you pull it through the plastic again, the whole page is blank again) - that would entertain her for a while on a long journey. And part of the fun was opening the package to see what it was.
Having a new Christmassy picture or board book might be good too. And there are loads of free seasonal printables from websites like Sata Update, Activity Village and DLTK that could be useful - from basic colouring pictures for small DCs, gradually getting more complex pictures and activity sheets as they get older. Santa Update also has games and some stories that you can print.
Our very first Christmas with DD, we bought a nice small hardback version of "Twas the night before Christmas" and that has been bedtime story on Christmas Eve ever since. (DD will be 13 this Christmas).
The thing about renting a cottage is to allow everyone have some space and gives a release from additional tensions due to the time of year and expectations that are often not met, as well as the expectations on you that you will be available for everything but sometimes you will need to prioritise a nap or something for DD.
We stay with family on every other trip during the year. And we have had to do Christmas in one family house in recent years (which just reaffirmed our decision to spend the € on a rental). Sometimes you really do just need space to close a door for 5 minutes peace, and want to read a book, or look at your emails, or scratch an itch - without it being everyone else's business and "but we need you to do this" or accusations of "and when will you be back to spend time with us? You've spent all your time with them"....when all you want is time alone.
Maybe you are more lucky with family and they don't have such expectations - but we found both of ours both got even worse once DD came along and we really needed an escape hatch!
And part of the way to sell it to family was that it meant we weren't putting pressure on them to sort beds and things, but we were able to organize to entertain them for X event and Y event, and it meant both families could come together on neutral territory (not 1 DM coming onto the "turf" of the other DM - Irish Mammies!) and enjoy each other's company (the 2 DFs both were very appreciative of the politics being removed and they could just enjoy a beer and chat about the GAA in peace!).