The country I live in (USA, you may have heard of it) once counted abundant, low cost labor as a comparative economic advantage. At that time, we exploited this advantage, which resulted in a sustained economic boom, accompanied by exploding output, and eventually the creation of a middle class. Our middle class then organized themselves and enforced much better working conditions. This eliminated our labor cost advantage, but we were able to make do with productivity improvements and a shift to services.
Imagine if, say, the UK meddled in our business in the 1880s and forced us to improve factory conditions prematurely. Our growth would have been slowed and the eventual creation of the middle class would have been delayed. A well-meaning effort to improve the lives of a few then would have hurt the quality of life for many later.
Those who criticize Chinese working conditions are either ignorant of economics and history or have an agenda to hold China back.
Further Reading:
http://www.hamiltoninstitute.com/the-problem-with-sweatshops-is-that-there-are-not-enough-of-them-discuss/