I have successfully used external linkages to Child Projects, stored in a reporting folder. At pre-determined intervals the Project Manager of the Child Project saves a copy of his (working) copy to the reporting folder, overwriting the one that already sits there. The external linkages are retained, not with standing any additional tasks that may have been added to child plans etc.
It's fairly easy to test this locally with a few simple test files, 1 x Master and a few Child projects (but ensure you have a different name for the working copy of the child MSP plan, and just save it over the file already stored in your reporting folder)