You’ve arbitrarily deemed it “indefensible”, and without citing a specific code of morality. I thought we weren’t turning ourselves into authorities here?
In other words, when faced with the reality of an immoral Amalekite people, who were doing all sorts of immoral things, I can give a reasonable moral alternative to massacring all Amalekite children. I believe it is moral to spare children from slaughter, no matter the sins of their parents.
If you prefer the OT approach, that's your prerogative. But my solution is objectively more moral.
Is it justifiable (without alternative) to slaughter Amalekite children, and if not, on what moral basis you make the claim.