Hmmm. Yab slightly u. My parents have never moved any of their small and toddler height trinkets out of the way and all their grandchildren have managed just fine. Their trinkets are 101 teeny ornamental animals that, to a toddler, are just perfect for playing zoo/vets etc with.
We managed the risk at their house by having a well stocked toy box so that held their interest rather than the trinkets. They didn't see anyone else touch them so generally once they had been told and guided away a few times, they seemed to just understand they weren't for playing with. I appreciate not every child will understand that and certainly my twins took longer to pick it up than anyone else did.
Pil on the other hand chose to put away their picture frames from the hearth and their China mugs on a slim stand. They didn't want to risk injury to the dc as much as they didn't want them broken. The problem was if we dropped round on an unplanned visit, because these things weren't normally there, they made a beeline for them while we grabbed them out of the way.
I would approach mil from the angle that you are putting her and her property first.
Eg s-mil, I moved the pretty, glass trinket box you inherited from great, great, great auntie Mabel from the low coffee table because I know how much you treasure it and would hate for dc to break it by thinking it's a toy.
It sounds better than 'I moved the glass, sharp edged trinket box because I don't want pfb to hurt their precious fingers on that accident waiting to happen'.
Of course those aren't the words you would use, but they may be how they are heard. Is she a step-mum that you've grown up with or were you two more distant, respectful but not close. I wonder if she perhaps feels you moving her property - you don't refer to it as 'Nannie and grandad's' is pushing her out of the family. Just armchair psychoanalysis. But maybe a thought?