others would find a way to create drama over whether their MIL gave their child their first taste of cake, or they should have known not to take duck feed to the park because they've stolen baby's first duck feed.
Exactly. All these OP's come from DIL's who expect grandparents to mind read. It's about communication for goodness' sake. If there are things you want to be your own 'firsts' then say so.
Also throughout this thread, people are totally twisting MIL's words. She didn't demand anything, she didn't insist on anything,
The mil has told me shes giving her some pjs on Christmas eve for her to wake up in and wear through the day.
FOR her to wear... Not that she MUST wear, or HAS to wear. There's no insistence, no assumption of rights. Just that it was her thinking when she bought the pyjamas. I see nothing in that that implies that she'll kick off if OP says, "that's lovely, but actually, I've got an outfit already that I'll change her into after breakfast"
Don't listen to those who want to think the worst of her, OP. There's no reason to be snidey, to lie, or to ruin the pyjamas by deliberately spilling things on them. Just be a normal human being, communicate what your plans are for the pyjamas with a smile and thank her for her gesture.