I think this was a one-shot, quite a few years old. HG is a teacher at Hogwarts and falls for SS at a masked ball, not know ing who he is. They conceive a child (and possibly meet up each year at the ...
It doesn't match your description 100%, but might the first one be Sweetest of All, by Loraine?
http://ashwinder.sycophanthex.com/viewstory.php?sid=622&i=1