Spoiler warning!
The children were being incorporated into the bodies of the aliens. In the words of the rather terse (and uber-creepy) alien ambassador:
They create chemicals. The chemicals … are good. We feel … good. The chemicals are good.
Basically, they were being used as prosthetic glands secreting a narcotic, which the aliens found pleasant. That’s why they wanted them. The aliens were shooting up, children. Somehow the (original 1965) children were preserved in their adolescent state, presumably indefinitely, melded into the alien’s monstrous bodies. The nature of their subjective experience was left undisclosed, but it is hard to imagine it was pleasant, nor that, being reduced to glands, that they had any sort of autonomy.
So yeah, being a cancer patient or otherwise unhealthy child did not appear to be any ‘advantage’, at least not from the perspective of the child, since presumably their suffering existence would only be extended forever. Hard to see how the parents would see this as a plus either.
I can think of a couple of differences:
The poor people during a famine at least have a fighting chance, if slim. Somehow, by hook or by crook, attain money or food, or leave for a region where there is no famine.
Also, a famine is a matter of public knowledge, which allows the possibility for a society to collectively (or fragmentedly) come up with a solution. In the torchwood scenario, [small spoiler warning] the true nature of the threat and the solution devised by the executive branch were being kept a secret. In fact, they were actively suppressing groups who were moving for alternative stances towards the alien threat. If it were public knowledge, the to-be-sacrificed class would at least have the option of revolting against the powers/system/‘algorithm’ which was mandating their extermination.