Cocoa/chocolate is an industry that happens to use a fair bit of child slavery, unfortunately (among other bad practices, I just single out that one because it's the one that I consider the most abhorrent)

Getting a fair trade certification requires a supply chain inspection, and in theory, would not be possible to get for producers which have child slavery in the supply chain. Though of course, supply chain inspections are complicated and I believe this is by no means perfect (nothing is).

Rainforest Alliance certified has nothing to do with ethics in treatment of the humans involved in the supply chain, just environmental impact.