Sure, but the argument FDR was putting was that he was doing it for the 'ethical' business owners who were supposedly forced to keep employing child labour beca

Sure, but the argument FDR was putting was that he was doing it for the 'ethical' business owners who were supposedly forced to keep employing child labour because others were still using it. No. The business owners could have and should have stopped doing what they saw as unethical by themselves. (I'm not arguing against anti-child labour laws here; I'm arguing against this reported argument for their introduction by FDR. And, to be fair, it's only the reported argument here that I'm responding to; it might not be a faithful representation of what FDR actually said.)