Think about it. If the parents needed to be natural born citizens, no one could ever qualify. Those in the who were in colonies or immigrated before or during the revolution, where not citizens of the US at birth, so they can't have been natural born. Thus their children would not have been able to have been either, and their grandchildren...
The same with those immigrating after the revoluation. They could be naturalized, but could not be natural born, thus niether could their children, grandchildren... down to the Nth generation.
Now it would be possible for the criteria to be that a Natural Born Citizen must have parents who were born citizens. But that's not what Vattel's "Law of Nations", or several SCOTUS cases say. They say the parents must be citizens.
But either way, born citizens or just citizens, BHO Sr fails the test, having been not only Kenyan born, but also never even a naturalized citizen of the US. He was a visiting student, not even a permanent resident alien. Thus BHO Jr cannot be a natural born citizen.
Or you could say it would take just a few generations be born before they would qualify. The way I wrote it, you as a naturalized citizen may not qualify, but offspring of your son or daughter would, provided they wed another citizen. The problem would correct itself quickly. And today there are natural-born in plentiful numbers.
I have read what was said in various founders documents, and the intent was to preserve the heritage of America by the natural-born requirement — why else would they do it?