Unborn war mage offspring the plan is explicitly intended to destroy along with the enemy soldiers.
Laying out the Bijanbi and Wall of Fire combination, Rosetta states plainly that it will kill the enemy and any potential baby war mages, and that if any survive that, there won't be many of them.
It is this detail, among others, that provokes Carl's reaction — he pictures what is about to happen, says "Oh. Oh fuck," and a sense of revulsion washes over him. The horror of the plan, and of what casting War Crime might do to Donut, is what drives him to give her an explicit way out.
Part of what makes the War Crime plan an atrocity in Carl's eyes.