I have no problem believing that Jefferson fathered Sally Hemings children. I think it is possible (and likely) and wouldn't be surprised if it was ever proven (which it hasn't been and never will be)
My problem is the continuing stance by many historians who speak of this as though it has been proven without a doubt and that Jefferson is certainly the father of her children. The "evidence" should be presented and when done the answer from historians to the question should be the same answer they give on thousands of historical topics...."We really don't know." Not even the DNA test that people constantly reference is 100% proof of anything. It certainly causes me to pause and lean a little towards the "Yes he did" side of things, but there is enough doubt (Randolph Jefferson is just as likely a "candidate") for me to refuse to take a stance one way or the other.
It's a fascinating mystery and will be a contentious issue for the rest of human history.