I would suggest you to get the list of pxCoveredInsKeys for all the subcases (child cases ) for your parent case. You can iterate it by using a symbolic index <LAST> which will retrive the highest index , take the pxCoveredInskey of that instance and use it for your usecase.
Let me know if it helps you.
Please find below the list of symbolic indexes that you can use while loop through the iteration :
<CURRENT> - Identifies the index of the current iteration <APPEND> - Inserts the element at a new position at the end <LAST> - Retrieves the highest index <PREPEND> - Inserts the element at the top <INSERT> - Inserts the element at a specific position that is indicated by the number