To loop all users from multiple people column, you can refer to:
It works for multi-value enabled lookup column, you can refer to the same way for the ...
Based on you description, I understand that you want to use flow on elevated privileges in the SharePoint Online.
Here're some references for you:
Elevated privileges in power automate.
How to use Flow on elevated privileges in SharePoint Online
Whether this issue occurs on all workflows?
1.Go to Site settings -> SharePoint designer setting under site collection Administration -> Check all the options -> Click ok.
2.Try to uninstall SharePoint Designer and re-install again with a windows restart.
3.Check ULS logs.
You could provide some screenshots of the parent list、 child lists and the workflow for further assistance.
If you want to update all the associated child items, you should add multiple update list item actions separately.
This issue may be caused by insufficient users’ permissions. Please check whether users have enough permissions.
You could try to create a workflow with elevated permissions.
Create a workflow with elevated permissions by using the SharePoint workflow
SharePoint 2013 workflow app step and app permissions