@jodie34,
I'm a bit like you, I like my displays and at Christmas, you are really wanting each area to have that "special" feel about it.. Not everyone can see that or care for it.. They live their lives differently.
I appreciate it's hard to see those displays messed with, un-appreciated for what they are, but we are not all the same remember.
I don't see any harm in taking cloth items of a Christmas tree to play with in fact i would have bought little things for the baby and had them on the tree, help Mum by bubs being distracted and help you all so there is more time to talk and enjoy each other's company...
I'd be at that door as soon as they arrive, saying "here, give me those" and take the bags and have a place where they belong for the duration...
And, I'd not worry about the bathroom, I wouldn't have Christmas decorations in there to start with, it's called compromise you know that people are going to use the bathroom and it's a small room.
I'd use areas that aren't going to be used by others to express the spirit of Christmas.
It's a few days of a whole year, where otherwise, what ever you display in your home remains that way, as you like it... Compromise where and how you are going to have your settings for next year...
We can be a bit OCD over things sometimes where "displaying" our homes can come into play and people moving things or putting things on top,.