Again, by definition genocide has no military purpose.
Israel’s military objective is to destroy Hamas. According to Western military doctrine (which Israel is capable of using), this objective does not require wiping out every last Palestinian. So it doesn’t matter what Israel “determines”, wiping out every last Palestinian is not permissible.
I think if war is justified, then killing children is justified because children are always killed in war. Personally I’m ambivalent about whether war can ever be justified, but I certainly recognize that most people think it can.