i) When Christians defend OT holy war, they sometimes justify the execution of child by appeal to original sin. Every human is guilty is Adam.
ii) Now, I think that's theologically true. However, I don't think that's the best way, or even the right way, to defend OT holy war.
To begin with, original sin is just as controversial as holy war. People who find holy war morally offensive find original sin morally offensive. When you defend holy war by appeal to original sin, that just pushes the argument back a step, because you have to defend original sin. You're defending one thing by appeal to something else you must defend. But in that case, what's the advantage? Why not defend holy war directly?
Someone might object that original sin is true regardless of whether critics find that appeal convincing. No doubt. But if you're going to fall back on an argument from biblical authority, you could just as well say the holy war commands are morally justifiable regardless of whether critics find the appeal to biblical authority persuasive.
iii) But besides the tactical problem, there's a substantive problem. The OT holy war commands and holy war accounts don't ground the ethics of holy war in original sin. So there's no reason to presume the Adamic guilt of children is a necessary condition to warrant their liability to be killed. Indeed, that argument may well be a blind alley.
iv) Let's consider a different principle. Take the case of an asymptomatic carrier. By that I mean a person who harbors a contagious disease, but is immune to the disease. The carrier is infectious.
Suppose the carrier is host to a highly contagious disease with a high rate of morality. Suppose the disease has a long incubation period.
The carrier may infect hundreds of people, who in turn infect hundreds of people, and so on. The disease is communicated at an exponential rate. By the time the disease manifests itself, it is unstoppable.
What should be done to the carrier? Through no fault of his own, the carrier is host to a deadly contagious disease. Although the carrier may be personally blameless, that doesn't change the fact that he poses a dire threat to public safety.
Should he be quarantined? That's unfair to the carrier. He's done nothing intentionally wrong. But even so, he puts the entire population at risk.
Moreover, is that a solution? Even in quarantine, even in solitary confinement, he's dangerous. He must be fed. Must be guarded. Any direct contact with the carrier is fatal.
Furthermore, he's a flight risk. What if he tries to escape? After all, his isolation is unbearable. How can he stand to be cooped up year after year?
v) Under the circumstances, would it be permissible to kill him? Should his life imperil a million other lives?
My objective is not to answer that question. I'm not making a case for that claim.
But unless you're a Kantian deontologist, it's not unreasonable. And I daresay most infidels who attack OT holy war aren't moral absolutists.
If, in principle, it is sometimes morally licit to take innocent life, then critics of OT holy war can't attack it on those grounds as a matter of principle.