Yes, we have heard all the critics about this song. Did Mary know about her Son Jesus and His being a Savior! Yes, she certainly did. The Gospel narrative regarding this story couldn’t be more clear (Luke 1:26-38).
But, keep in mind that songs, just like movies, take artistic liberties from times to drive home another point. In this song, I personally believe the modern song’s writer was trying raise the issue of the wonder of a God-man coming to earth through the life of a human woman. No human could or woman design it this way. When we sing “Mary, did you know” we are singing about the astonishment of such a coming.
What a miracle!