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.