You raise a good point. The plural "citizens" implies that both parents would have had to have been citizens for the child to be automatically considered a "natural born citizen".
Per NPR, Cruz's father did not become a US citizen until 2005, meaning that, at the time of Cruz's birth in Canada, he only had one parent being a US citizen.
The Naturalization Act of 1790:
The Act also establishes the United States citizenship of certain children of citizens, born abroad, without the need for naturalization: “the children of citizens of the United States that may be 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”.
___________________________
I did a little checking. Somebody is not playing straight. It looks like the quote I posted left out the last clause which I have reinserted (beginning with “provided”).This rules out Cruz and Obama, does it not? I had copied it from a previous poster who was maintaining Cruz was eligible. The point I am making refers to what the Founders had in mind when they used the term “natural born citizen”. Obviously, naturalization has been changed since 1790.