I have always suspected that Tom Woodson (the alleged oldest son) was not Sally's son either- the one Hemings child who gave an account of his parentage, Madison, claims that Sally's first child died in infancy.
One of my college professors was on the committee, and he loved to find out bad things about Jefferson. However, even he could not conclude that Thomas Jefferson was the father. He didn't think Jefferson physically had it in him to father ANY child when the alleged offspring were conceived. I'm less certain and think it possible.
Given the fact that I think President John Tyler remarried at 54 and had seven children in his fifties and sixties with a wife thirty years younger I also don't believe that it was impossible for Jefferson to father children up to age 65.
I tend to believe that he and Sally had a relationship, based on the fact that she only conceived when he was at Monticello, this during the years in which he was serving in Washington's cabinet, as Vice President, and President. It has not been document that Randolph or any other Jefferson was there at all the times she conceived children. In fact, no other Jefferson was advanced as suspects until after the DNA tests- before Jefferson's family and historians insisted that Jefferson's nephews the Carr brothers were the likely fathers- too bad they were excluded by the DNA tests.