The killing of children is widelly deemed the most immoral kind of murder, because there is no way it can be spinned as the killing of people guilty of anything or a self-defense: children are almost by definition innocent and incapable of defending themselves or overpower an adult, especially young ones - it takes a special kind of individual (namelly, sociopath or psychopath) to not only willingly take human life, but even that of those who are guaranteed innocent and totally incapable of defending themselves, much less attacking.
This actually is reflected in the numbers of murder victims, were children are killed in a far, far lower proportion of their numbers than adults, and ditto for war casualties: adults on both sides usually will go a lot further in protecting children, even not their own, than they will for adults.
It is surprising that in your moral framework you do not see the murder of children as an especially cold and calous kind of murder.