Add layer to all master pages

The perhaps better way to setup Parent Pages is to Base them on each other. So your top Layer is a Parent of the Parent pages, and the rest of the 'parent' pages are 'children'

That way you just add it once to the upper most 'Parent' parent page and it's added to all the subsequent 'Child' parent pages.

But I feel it's too late for this workflow.

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I'd start by creating an Object Style for the object you want 

In the Object Style you can select it's size, colour, position etc. 
So in the future you can edit the Object Style to increase/decrease/shift etc. the object without having to visit 50 master pages. 

There's a script here you might try to copy the item to other pages - haven't tried it 
But might work
https://creativepro.com/topic/pasting-on-multiple-pages/

Or you might find a script somewhere that can copy to Parent Pages.

Honestly, it's quite quick to copy once from the 1 master page then double click into each Parent Page and Paste in Place.