If a child were well-behaved, it was safe, of course. But disobedient children could be turned in by their parents. They'd be given a math test, and if they didn't do well, they would be "eliminated".
Eventually, a High School Math Teacher climbs into the Child Collection Van and professes that he doesn't really understand algebra that well and asks them what they will do about it.
The point: all such yardsticks are arbitrary and wrong. Human life is sacred, and it is not hard to identify "human life". Only evil people pretend that it's an intellectual challenge.
Well said.
And I would like to read the story that you mentioned.