OK, folks, let’s slow down and look at the practicalities. I’m not an expert on immigration, but I am an immigrant. Please feel free to correct any mistakes I make. I’m posting from experience, not knowledge
First, a significant number of children born in the United States to non-US citizens are the children of legal immigrants, including my own little brother, to account for my own interest in this. Under most circumstances, legal immigrants are not eligible for citizenship until they’ve lived in this country for 7 years, which is an ample time span for children to be born.
Second, as I understand it, there are two documents which can prove US citizenship on their own: a birth certificate and a Naturalization certificate. If we decide that being born in the US is not adequate for citizenship in and of itself, then this complicates the matter of birth certificates. Do they vary from city to city or state to state in the US? I gather they do. At any rate, we’d need to modify them to indicate whether the baby is considered a citizen of the US at the time of his or her birth.
Third, if being born in the US alone is not sufficient for citizenship, would one or both parents have to be citizens and how would this be determined? If it’s one, what’s to stop a woman from lying about the baby’s parentage? We did, after all, have a thread about just this in MPSIMS recently. In some states, including mine, a woman’s husband is presumed to be the father of her baby. If a woman who’s a legal resident married to a US citizen has a child, here the child would be presumed to be her husband’s child therefore granted US citizenship, assuming only one parent must be a citizen for citizenship to be conferred on the child. However, what becomes of the child’s citizenship if, sometime later, the child’s biological father is determined to be someone else who isn’t a US citizen? For that matter, if the law had required that at least one parent be a US citizen, what’s to stop someone like my mother from claiming the father of her child isn’t her (legal immigrant) husband, but the guy next door who was born and raised in the US?
As I said, I do have a bit of a personal stake in this issue, but it looks to me like it would be opening a rather large can of worms and I seem to recall some sage advice about never opening a can of worms unless one plans on going fishing! 
By the way, if you think the scenarios I mentioned in my third point are unlikely, I can give you a cite to a recent case in which custondy had to be determined. The parties involved were a surrogate mother, a sperm donor, an egg donor, and the couple who were trying to have the baby in the first place! :eek: