If you enforce labor laws in developing countries, the cost of production will be high enough that industry doesn't develop. At that point, the people there have no industry and are much worse off (e.g. a sweatshop is better than subsistence farming).
A better argument is 'labor choice'. Individuals need to have the ability to choose their circumstance. If they can choose, and choose freely, over time the whole society can ratchet up their standard of living by making these choices at the margin.
If the reason is ethical than why would we support sweatshop labor, or any other exploitative labor?
What exploited people need is not sweatshop jobs but access to capital and education. Invest in them, do not exploit them.
Investment in people is a red herring. You can give people choice and invest in them. Though guess what: developing countries (which is who we're talking about) don't have money to do the latter.
Or, to put it another way, if two people wish to trade, it must be in both of their interests. If I, as a westerner, go to a third world country and buy a bunch of stuff that from my perspective is cheap but from their perspective is expensive, we both end up happy. What's so terrible about that?
(Of course where it falls down is where labor is cheaper because of poor worker protections and exploitative practices, in which case it constitutes an end-run around the labor laws of the richer party. But they are the ones that should be upset, not the poorer party, whose labor laws would be terrible regardless of exploitation)