It came down in our family as, "suns." And it does make sense- as it is the Christmas star (the Morning Star) that is being addressed. I think that the "mistake" was made long ago, and in the opposite direction (someone sang,or printed, "sons" instead of, "suns"). As I pointed out in another thread, last year or so, Lucifer was the one referred to as, "Son of the Morning," so wouldn't it be odd for a composer of sacred songs to write the line so?