All right, this is funny.
One of the earliest Christmas hymns is Prudentius’ “Hymnus VIII. Kalendas Ianuarias” (ie, “Hymn for December 25th”). And the 14th stanza asks the musical/poetical question:
“Sentisne, virgo nobilis,
matura per fastidia,
pudorem intactus decus
honore partus crescere?”
But the trick here is that the question is, literally,
“O noble Virgin, do you understand,
During your delicate state come to its time,
That your intact dignity of modesty is to increase
With the honor of your offspring’s birth?”
So in this case, the question is not, “Mary, did you know your Son would be the Savior?” but instead, it’s “Mary, did you know how impressive this was going to be, for you?”
The point is that, despite the weird squeamishness of some of today’s Catholics and other Christians about it, the miraculous birth of Jesus was always supposed to be just as miraculous as His virginal conception in Mary’s womb. Mary’s title of “Ever-Virgin” (Aeiparthenos in Greek) is ancient. So Prudentius, our early Christian poet, is of course going to be concerned with (rhetorically) asking Mary if she knew she would remain virgin during and after Jesus’ birth.
But he assumed she’d already gotten the picture on Jesus being the Savior, the Messiah, and God, because he assumed Mary had paid attention to Gabriel’s announcement. I guess today’s songwriters do not assume this.