Such values are entirely relative, within a society hell bent on increasing its numbers to conquer worlds/economies/cultures, favouring breeding members of society is logical and to be expected. I aim for a the opposite society, a society that grows in quality not quantity, a society which no longer uses women's wombs as means of production.
You might be overboard in the fact that the killers will most likely kill both of you, and saying "kill him so he will stop making babies" is just really a selfish excuse. But who am I to judge on what people say to try to extend their lives.
Though now that you mention it, it is rather offensive that they think they are worth so much more for having kids.
Somehow I'm seeing a similarity between this and "give me a lesser sentence because I believe in God". Because it's pleading based on something that should have no bearing on the subject at hand.
I'm not sure I'd view "Don't kill me I have kids!" as overtly declaring themselves more valueable than the childless per se. I'd imagine that a parent who cares about their children would be very distressed (understatement) at the idea of them having to suffer the loss of one or both parents, particularly if it might result in their being thrust into the gentle and loving arms of the foster care system. On the practical level, when children become orphans (or screwed up from losing a parent) I believe society expects to pay for it one way or the other. If they grow up maladapted for lack of the parent that was brutally murdered and land in prison, say, it costs us money as a society. If one of us dies our families would be traumatized, but theoretically they aren't reliant on us to survive whereas children would be, and they aren't in the middle of important mental development that might be seriously impacted by our death. At any rate I doubt they're actually thinking about any of this in the heat of the moment, and I don't think I'd hold anyone responsible for what they say at the wrong end of a gun.