Because the Palestinian children had nothing to do with the killing of Israeli children? What you're describing and explicitly trying to justify here is collective punishment of all of the two million Palestinians in Gaza (more than half of whom are children) for the crimes of (by Israel's estimates) about 3,000 Hamas terrorists on 7 October.
What you're articulating constitutes a war crime under the Geneva Convention and that's exactly why the ICC is getting involved.
Let me try putting this another way. The population of the US state of Nebraska is about two million. Every year, there are about 6,000 violent crimes committed by Nebraskans. Should every Nebraskan be collectively punished for the crimes of those few thousand Nebraskans?