Has it really been proven that free trade is good for poorer, less developed countries though?

Many countries in Latin America opened their markets to free trade with the US in the 1980's for example, but Latin America continues to have the greatest disparity between wealthy and poor people, with 15% of the population controlling roughly 85% of the wealth. Over 1/3 of the population lives in poverty.

I also remember reading about how Under the Central American Free Trade Agreement, Nicaragua was granted special trade privileges that will allow abusive factories like King Yong to export 427 million garments to the U.S. duty free.

So if countries aren’t required to improve working conditions and we all know how bad many poorer countries working conditions are, how good is it for the poor?