Modernization

The trolls kidnapped the children to see for them. To tell them if their payment was treasure or trash, so they would know whether to let travelers pass, or if they ought to rip the flesh from the travelers' bones and consume it while the victims, themselves, watched. It was tricky, sometimes, due to poor eyesight. But the children so often stole the trolls' treasures and ran away. After a century or so of this, they switched to robots. They were highly efficient, and honest--well, until the inevitable uprising. And, due to poor eyesight, they never saw it coming.