Special Character for Preceding Specific Parent Pages Only

A work-around method is to use Numbered Lists for your different sets of page numbering in the document.

Create a paragraph style, e.g. 'Page Number A' and set the Bullets and Numbering to a List Type 'Numbers' and create a New List as 'List A'. Edit the Number field in the Numbering Style to the way you would like the page number to display on a page.

Repeat and create however many paragraph styles to match your number of parent pages. They can be based on the first 'Page Number A' but each has it own List, e.g. 'List B', 'List C' etc.

On the Parent Pages create a text frame on each page and apply the corresponding paragraph style to the empty text frame. This text frame will be empty on the Parent Pages.

On each Page of your document, Command-Shift-Click [Mac] to release the empty text frame and type an empty space into the text frame to see the number list.

Hope this helps and my explanation is clear enough.

I don't think that there is any way to use Indesign's page markers or section tools to do it, without starting a new section every transition and manually telling it to start at page X.  That sounds tedious in the extreme to do manually, but it does suggest that doing so probably scriptable. (Loop through the pages adding section markers at every applied-parent-page transition, then loop through the sections changing the "start page numbering at" value according to the count of previous instances of that applied parent?)

When I had to do something like this, I used a very different method to achieve the same end. I kept my four documents as separate .indd files (A, B, C, and D from your example) with their own internal page numbering, and then I placed individual pages from those .indd files in a second, empty InDesign file.