Sorry, I’m still having trouble understanding what the issue is.
Are you saying that every time the child pipeline runs, it’s overwriting the same file? Then why not just change the file name so that it’s unique per execution? There are many ways to do that, such as appending pipe.startTime or pipe.ruuid to the name.
Also, even if the file is being overwritten every time, why can’t you just look at the latest version written?
You can also just validate the child pipeline by itself to test it by setting the pipeline parameters to default values that test the functionality you want to test.