A lot of people appear to believe that if you simply outlaw something you don't like, you've solved the problem. Child labor is a case in point: people don't like children working in factories, so they figure that simply outlawing it is the solution. And if that's the case, then the problem can be attributed to a lack of such laws in the first place.
But the world isn't that simple. Children didn't end up working in factories because it was legal, they worked in factories because the alternatives were worse. That's something those arguing for child labor laws still haven't come to terms with. If all you do is outlaw child labor, then you're just forcing them into those worse alternatives. And if you provide better alternatives, or better alternatives are available, well, you don't need the child labor laws to make those children pick those better alternatives. But the problem of child labor is NOT a problem of a lack of laws prohibiting it, it's a problem of a lack of better alternatives.