They've already been attacked (and successfully protected). So, avoiding psychological damage in the children is already impossible. This is a reflection on whether letting the kids do the execution of prisoners themselves is cathartic. Or should they remain helpless and cowering. The entire thing is the MC reflecting that he's not entirely happy with the situation but completely understands the need for it.

That, and the adults literally get 1-2 XP as most of the prisoners were low level, but swarming. The same formula in reverse gives the kids a ding every 1-3 kills, at least the first few times. While these aren't child soldiers and aren't meant for attacking, it does keep them from being taken out by a stray arrow or area effect while the women are defending them. And that is a very real concern because of the three enemy factions that have currently been introduced, all of them will specifically target the young if given the chance.

Also, 10-11 specifically crosses a tier line. No amount of XP grinding alone is sufficient for this. A young person (14 year olds are adults in the setting) would not get past 10 even with power leveling. The fame/infamy requirement would wall them.

As they were processed like an assembly line, a few had tried raising dissent by claiming this was a genocide or some such. Of course, it was not, and even if it were, that wouldn't constitute a compelling argument as you could not convince any sapient worth anything that killing their enemies was wrong. What in the Earth were you supposed to do with them, see things from their perspective and pretend their arguments are legitimate and reasonable? That's for stylistic disagreements, such as "What is your favorite flavor of ice cream?" and is entirely off the table for existential threats such as Life VS Death or Freedom VS Slavery.

The mere fact there were soy-based lifeforms on Earth who would unironically make such a claim was proof that, with very few exceptions, our cultures were incompatible. Or, more accurately, any culture was incompatible with a lack thereof...