Is Morality Just a Social Construct?
Do feral children prove that morality is just a social construct and not instilled into us at birth by God?
Not at all. They only illustrate how our human nature would respond in extreme circumstances. Our understanding begins with our bodily senses, and so if we are deprived of rational discourse and experience our whole lives, it will be most difficult to develop them later. St. Thomas Aquinas teaches that it is possible for someone to be ignorant of the natural law because of an impoverished formation. The natural law is simply our reason's appreciation of what is to be done and what is to avoided.
God governs us through the use of our reason, and this moral reasoning can be more or less accurate according to the quality of one's experience and the instruction one has received. If one has been raised without the use of language and outside a human social context, then his moral reasoning would be nil or very rudimentary. One would have to begin with the Golden Rule and move on to the other things built upon it. As it is, most cases of feral children are controversial.