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assuming you're referring to this:

CHAP, III.—An Act to establish an uniform Rule of Naturalization.(a)

SECTION 1. Be it enacted by the Senate and House of Representatives
of the United States of America in Congress assembled, That any alien,
being a free white person, who shall have resided within the limits and
under the jurisdiction of the United States for the term of two years,
may be admitted to become a citizen thereof, on application to any common
law court of record, in any one of the states wherein he shall have
resided for the term of one year at least, and making proof to the satisfaction
of such court, that he is a person of good character, and taking
the oath or affirmation prescribed by law, to 'support the constitution of
the United States, which oath or affirmation such court shall administer;
and the clerk of such court shall record such application, and the pro-
ceedings thereon; and thereupon such person shall be considered as a
citizen of the United States. And the children of such persons so
naturalized, dwelling within the United States, being under the age of
twenty-one years at the time of such naturalization, shall also be considered
as citizens of the United States. And the children of citizens of
the United States, that maybe born beyond sea, or out of the limits of
the United States, shall be considered as natural born citizens: Provided,
That the right of citizenship shall not descend to persons whose fathers
have never been resident in the United States: Provided also, That no
person heretofore proscribed by any state, shall be admitted a citizen as
aforesaid, except by an act of the legislature of the state in which such
person was proscribed,(«)
APPROVED, March 26, 1790.

source: http://legisworks.org/sal/1/stats/STATUTE-1-Pg103.pdf

this section refers to naturalizing the citizens within the NEWLY FORMED country. it was also repealed in subsequent acts over the following 7 years starting in 1795.

as is, this provision was to deal with the situation at hand at the time. two people in the country were newly defined as citizens and their children would be defined as natural born citizens, even if they were born in another country at the time this document was approved. this is a 'one off' situation needed to address the issues of the newly formed nation similar to the "at the time of the Adoption of this Constitution" phrased used in A2S1C5 of the Constitution.

either way, another thing to note is the plural used... "children of citizens of the US". this again implies two citizen parents needed to establish natural born citizenship.