(Apparently it is polite to note that this article contains "spoilers," or revelations about the plot of the works discussed, in this case Children of Earth, the third season of a BBC Doctor Who spin-off called Torchwood.)
An alien race known as the 456 comes to earth and demands 10% of earth's children - or they will destroy the human race.
What do they want with the children? A child previously abducted by the 456 is shown partially dismembered and physically attached to an alien's body, puppet-like. The child feels no pain and will never die; his eyes gaze out with a vacant, vaguely pleading expression. Why do the aliens do this? The human children produce chemicals that the aliens find pleasurable.
The situation is one of raw horror. A person of normal empathetic capabilities will find it absolutely horrifying for a child to be used in this way - cut up and attached to an alien to live forever as his drug factory. In the television show, a government worker who is informed his children will be among the 10% sacrificed to the alien overlords kills his children (and himself) rather than hand them over to this fate. Many of us might share this reaction.
What if, however, instead of kidnapping existing children, the alien could breed its own human children (in vats, say) for this purpose? Would that be wrong? The answer to this question gives us insight into the morality of creating children under normal circumstances.
One objection to the 456 creating children to use as drug factories is that although the children will not "feel pain," they will suffer a horrible, endless existence. But horrible by whose standards? If the 456 does not create the children, they won't exist at all. Isn't a painless, eternal existence as the appendage of an alien better than none at all? Can we even compare the two? What standard could we use to decide whether a proposed existence is "too horrible"?
A second objection is that it contemplates using children for very selfish reasons. "Child-as-drug-factory" is about as selfish as you can get in terms of motivation for creating a child. But are ordinary human motivations any less selfish? We do not ordinarily inquire into motives for creating children. Should we?
The proponent of procreation must explain, I think, why it is wrong for the 456 to create children to use as chemical factories, but not wrong for ordinary humans to have babies for such motives as personal enjoyment and a feeling of immortality.
See also The Austrian Basement and Beyond for a similar thought experiment.