[Logophile]

The writer had no kids?

Quite possibly he or she didn't. However, it is clear from the rest of your post that you do!

[agedav]

Why a newborn might not cry (from a father of 5 and grandpa of 13):
1. He was tired from all the work of being born in the normal manner.
2. His mother had wrapped Him tightly in cloth to help Him feel secure, just as is if still in the womb.
3. He was in the soft hay in the manger.
4. His mother was with him.
But, indeed, Franz Gruber wrote the song many years after the event. Perhaps he did not know what really happened. Perhaps it is beautiful poetic license.