Shirt consumers don't have much of a choice. They can only buy what's for sale (and in their price range). And then, how can they be sure if a shirt was or wasn't made by child labor? How would an individual consumer's behavior lead to ending child labor?
According to the article, Facebook execs understood what the product was doing, and, while they have the ability to stop it, don't. Maybe I understand what you're saying if we're talking engineers/middle managers, but that's a boring conversation. The buck has to stop somewhere.