I believe the idea was to show that Herod was responsible for this crime.
I think you missed my point about both the mosaic and the carol:
Herod, the king, in his raging,
Charged he hath this day
His men of might, in his own sight,
All children young to slay.
And looking at the mosaic, there is Herod on the dais watching the murderers do their work.
Yet from Matthew:
Then Herod perceiving that he was deluded by the wise men, was exceeding angry; and sending killed all the men children that were in Bethlehem, and in all the borders thereof, from two years old and under, according to the time which he had diligently inquired of the wise men.
I'm just saying it's artistic license; the mosaic and carol present the same picture, Herod the evil sat and watched the butchery. This is not supported by the verse in Matthew.
Happy New Year!
Bonehead