"Whether and how the use and control of subcontractors would be affected by
allowing the third-party testing of component parts."
This is the hardest question for me, because I don't contract out, and have no experience with the manufacturing process outside my own basement. So here goes.
A quality control system would have to be put in place that may include randomizing testing and implementing stricter quality control processes. Instead of checking the first few from a production run, a manufacturer might pull and spot check somewhere down the production line. That way if the contractor has substituted a tested component for another, a check against the original components could be performed. In the apparel industry, this could be performed by a visual inspection compared against the original compliant components specified by the manufacturer.
As for toys, or other products, this might be harder to determine. Whether a certain paint has been swapped for another, might be difficult to tell without doing random unit testing. However, if you did do unit testing on one of the first few from the production run, and then later on a switch was made, unit testing wouldn’t have made that product any safer versus component testing. It really comes down to random quality control.
I welcome all of your comments to help me formulate these responses, as I'm pretty sure I'm not aware of many of the complicating factors that many businesses face.