The centuries-old scourge of child labor, which spread in recent decades as the world's factory floor shifted more to developing nations, appears to be waning as crackdowns on underage labor start to bite.

That conclusion, detailed in a United Nations survey released Monday and echoed in interviews with activists and officials in several developing countries, offers a rare piece of good news after a year of mounting criticism of conditions in the world's industrial sector. The criticism intensified after the April collapse...