There have been many similar situations historically - food supply was more or less equal to food demand, so if food supply got suddenly lower for whatever reason, there wasn't enough food for everyone, and some people had to die.

The usual algorithm was that the poor people would be priced out of the food market, until enough of them died to restore Malthusian equilibrium. Most of the dead would be children.

How is that morally different from the situation described in the post?

What if the children sacrificed are the ones who would expected to quickly die anyway? Young cancer patients, for instance, or any chld born with a defect that would have led to a very early death without modern medical care.

Their suffering would be diminished since it wouldn't last as long, and as a dark "plus", their parents were already facing the prospect of the child dying young.

I didn't see the show so I don't know if there was a caveat saying they had to be healthy children. Incidentally did the aliens describe the fate worse than death at all? Is it torture, or more like Borg-assimilation?

Lesswrongers will be encouraged to learn that the Torchwood characters were rationalists to a man and woman - there was little hesitation in agreeing to the 456's demands.

Are you joking? They weren't rationalists, they were selfish. There is a distinction. They were looking after there own asses and those of their families (note that the complicit politicians specifically excluded their own family's children from selection, regardless of 'worth').

children - or units as they were plausibly referred to

What do you mean by 'plausibly'? They were referred to as units in order to dehumanize them. Because the people referring to the children as such recognized that what they were doing was abhorrently wrong, and so had to mask the fact, even to themselves, by obscuring the reality of what they were discussing: the wholesale slaughter of their fellows.

... governments paying attention to round up the orphans, the refugees and the unloved - for the unexpectedly rational reason of minimising the suffering of the survivors

That's laughable. It had nothing to do with minimizing suffering, that was a rationalization. They were doing it for the same reason any government targets the vulnerable; because there are few willing to protect them and argue for them. It was pretty clear if you watched the show that the children being targeted were hardly 'unloved'.

You can't consider the scenario without considering the precedent that it would set. The notion that there are wide swaths of the population -- children, who've never even had the opportunity to truly prove themselves or do much of anything -- who are completely without worth and sacrificeable at the whim of the government is untenable in a society that values things like individuality, personal autonomy, the pursuit of happiness and, well, human life! They would not be saving humanity, they would be mutilating it.

The poster failed to mention that the sacrificed children were being sentence to an eternal fate which worse than death.

And there is a difference between the actions of the government and the actions of the main character. One of them was fighting the monsters. The others were the monster's business partners.