moral facts are facts merely about the human social order, about human nature. They are facts about human brains, human bodies and terrestrial biology and our terrestrial environment and the particular facts of our situation in this corner of the cosmos. Is it immoral for Martians to kill Martian children? I have no idea. But I know that it is immoral for humans to kill human children
All you get with this line of reasoning is descriptive ethics, as opposed to prescriptive ethics. That is, you can indeed demonstrate that human beings do in fact hold such and such ethical views. You cannot demonstrate that people should hold such and such ethics views. When people are discussing whether or not some given action is ethically permissible, they typically are not debating whether human beings do in fact find it to be ethically permissible, but rather whether human beings should find it to be ethically permissible. Prescriptive ethics is the problem, not descriptive ethics - which can be settled by conducting a survey.