I hate this song. Did Mary know who her son was? The multiple angels popping in and out might have given her some clue.
In the sixth month, God sent the angel Gabriel to Nazareth, a town in Galilee, to a virgin pledged to be married to a man named Joseph, a descendant of David. The virgin's name was Mary. The angel went to her and said, "Greetings, you who are highly favored! The Lord is with you."
Mary was greatly troubled at his words and wondered what kind of greeting this might be. But the angel said to her, "Do not be afraid, Mary, you have found favor with God. You will be with child and give birth to a son, and you are to give him the name Jesus. He will be great and will be called the Son of the Most High. The Lord God will give him the throne of his father David, and he will reign over the house of Jacob forever; his kingdom will never end."
So yes, Mary knew. Good grief. This song is dumb. I guess it was designed to play on the emotions of mothers. That's hard to do.