Morality usually comes from some sort of first principles. Some are religiously inspired, but I think it's much better to start with one or more moral philosophers.
If we simply put it to a vote, then we would likely conclude that slavery was moral in the 18th century.
Justifying the death of civilians implies justifying the death of children, because children are a subgroup of civilians.
Likewise, the ICC bans "intentionally directing attacks against civilians". They do not specify children. Do you suppose that means directing attacks against children is legal according to the ICC? Of course not, because children are a subgroup of civilians.
Finally, I am not your research arm either. You asked me the names of relevant philosophers, I provided them. If you have follow-up questions about their ideas then I applaud your curiousity but you should probably just read what they wrote.
I know you want to pretend that children are not a special class, but there's a reason we treat children differently from adults and the reason that I am specifically talking about then since you are saying their deaths are justifiable.