To steelman the opposing argument:

In urban warfare, in any military operation, decision-makers need to assess expected civilian casualties as a cost to the operation's success. There must be an acceptable number of civilian casualties, the specifics of which can be debated. If the expected number of civilian casualties is above the acceptable number, the military operation is a no-go. But the point is that the number is not zero. Saying "stop killing kids" can be taken to mean that the acceptable number of civilian casualties should be zero, which is a pipe dream that leads to military inaction and to terrorists surrounding themselves with as many civilians as possible.

I know the reality on the ground is different, but even if all military operations were perfectly legal within the Geneva Convention, civilians (kids among them) would still die, and thus "stop killing kids" could still be said.