So, I'm admittedly in the camp of "younger lives represent less resource investment. I am not against the de-abstraction of why human life has worth.
But this shit is just fucking stupid, for various reasons:
Children are 'innocent'. They have not done any thing that could possibly be construed as being ethically consenting to such an act. They have harmed nobody... They don't even understand harm. They have not consented to the risks or costs of a free life even to the extent that a teenager does. To violate the consent of a potential person is to spoil an unspoiled resource.
If a child survives such an attack, you have put a scar on the adult to an extent that it will create costs for all the rest of us; costs well justified by their existence, but costs nonetheless.
If a child does not survive the attack, you fail insofar as pissing on the biological and instinctive elements which act as the largest part of thr barrier between wanton behavior and good social action. When you can kill and moreover easily accept the killing of an innocent, you have become the most loathsome kind of monster, someone nobody ought trust in their society.
There is no way to harm a child without committing the sort of act that warrants drastic action against you. There is no context in which it is "more acceptable" for a child to be killed than an adult.
Yes, sometimes die, and the calculus of that says a child's death, in the instance of mere tragedy, is in general less grievous harm to society than the loss of an adult. But when there is intent, that intent is truly the darkest and most vile act that can be imagined.