Right click/Properties/Child ID

What's the best/easiest way to change this?  I need to modify the child report for this union report but don't want to impact where it is used in other reports.  

You would need a unique copy of the subreport.   In the SIR Advanced course copying union reports for nonfinancial reports is covered.  In the SIR Financials course copying union reports for Financials reports is covered

You can export the report. Then I would suggest creating a new folder in Report Manager and importing the report in that folder. You will then be able to modify the child report and it won't affect anything else because the only thing it is linked to is the imported report. FYI, if you are only modifying the subreport in Report Manager, then this is all you need to do. However, if you need to modify the container in Connector (deeper modification) then upon import, when it asks you if you would like to use the existing Container make sure you choose 'No Create a New One' and give it a unique name like adding V2 at the end. That way if you go into Connector to do deep modifications, you are not altering a container that affects other reports.

You can export the report. Then I would suggest creating a new folder in Report Manager and importing the report in that folder. You will then be able to modify the child report and it won't affect anything else because the only thing it is linked to is the imported report. FYI, if you are only modifying the subreport in Report Manager, then this is all you need to do. However, if you need to modify the container in Connector (deeper modification) then upon import, when it asks you if you would like to use the existing Container make sure you choose 'No Create a New One' and give it a unique name like adding V2 at the end. That way if you go into Connector to do deep modifications, you are not altering a container that affects other reports.