Abigailanne, I agree with PPs, especially Faye. IMO, DS and DIL are behaving in a very immature way, but they may feel as if you're saying without actually saying it, "We love GD more than DS; she's more important to us than anyone else; and/or we favor DD and SIL cuz they gave us a GC."
Balancing the scenario, by having more photos up of DS and DIL, if you don't already, as Faye suggests, may help alleviate the problem. Not that anyone has the right to a say in anyone else' decor. But if you want to try to ease their jealousy, adding a few more pics of them may be easy enough and worth giving a try.
It also may help to even out the number of Christmas presents a little more (if that's an issue). Also, perhaps paying more attention to DS and DIL when GD is around, asking them questions about their lives, and so on (again if you don't do that already).
I know it can be hard to focus on anyone else when an adorable child is around, especially if you don't get to see them all that often. So maybe DS and DIL are on the right track and it's better if they're not there when DD and family are. Sad for the sibling and uncle/niece and aunt/niece relationships, I know. But it may be necessary.
But Christmas is a long way off. If you manage to balance out the photos and such, perhaps they'll change their minds about not joining you for it, as we get nearer to the holliday(it's still a long way off). But if not, I think you should just let it be. Maybe see them, separately, on an alternate day, during the Christmas season. Or enjoy other events with them when DD and family back in Australia.