Maybe a better approach than stating emphatically and declaratively that someone is not eligible based on YOUR interpretation of supporting documents and the thought processes of the Founders, instead of what they chose to write down in the Constitution, might be to state that it is something that needs to be clarified by Congress and the Supreme Court.
But let's be clear, according to US Law and the US Constitution, as of today, Ted Cruz is eligible to be POTUS.
Let's also be clear that according to the documentation left behind by the legal experts of the early United States, "natural born citizen" absolutely DOES NOT require both birth on US soil and citizen parents. It only requires that a person be BORN A CITIZEN, which is exactly what Ted Cruz was.
This was explicitly stated by James Bayard in his Exposition of the Constitution, and that exposition was approved by the legal experts of his day. It was also used to teach the Constitution in our colleges and seminaries.
Here's the basic problem with birthers. This is the fundamental fallacy.
They start not by going and reading history, law and the Founding Fathers and early legal experts, with anything like an open mind.
They start with the conclusion. Then they go looking to "prove" it.
Or, at the very least, they start with a THEORY, that they then go looking for evidence to "prove."
And whenever evidence pops up that contradicts their theory, they go to all kinds of contortions to discredit it, twist it into saying something it never said, or flat-out deny it.
That's not how history works. That's not how competent and honest people process history. Competent and honest people begin by saying, "Let's go see what the Founding Fathers and our experts in the law have had to say." They then carefully and objectively weigh the quality of the evidence in each direction - if they have more than one theory of what something meant - and they go with what the evidence, interpreted in context, tells them.
So they derive their THEORY from the evidence. They don't come up with a nice-sounding theory and then go try to "prove" it and ram it down other people's throats even though it's in extensive conflict with the evidence.