@BillRM,
That is only true if the child enters the US with the his/her parents after having been born outside the US. For instance if it is only a trip they took and they had a baby outside the US. If the child was born outside the US and legally resides outsides the US, the parents have to apply for a naturalization of the child. Also, the child has to be in the US for the naturalization process and recite the oath of allegiance unless the child is too young. Also the parents had to have been in the US five years prior to the birth of the child, two of which after the age 14.
Consider unless Snowden comes back for trial he will still be outside the US, if they have a child, there are a lot of steps the mother will have to go through. But once she does, I reckon she go back. Unless of course they wait too late to have a child, then I am not sure the conditions will be met.
Also, I just now thought of it, if Snowden's girlfriend is not legally living in Russia, but only visiting, then, I think it is all simpler from the US perspective.
(all the information for the above I left a link for yesterday)
Anyway, this was all speculation on an event which has obviously not happened, it was a boring Sunday afternoon and I was just trying to get a conversation started.