John Calvin Commentary Isaiah 13:18

"And [their] bows shall dash the young men in pieces; and they shall have no pity on the fruit of the womb; their eye shall not spare children." — Isaiah 13:18 (ASV)

And with bows they shall dash in pieces the children. Some interpret it as, they shall cut. They think that the language is exaggerated, as if they used the children of the Babylonians instead of arrows, and then dashed them to the ground, so that they might be broken with greater violence.

But I prefer to take a more simple view of the words: the cruelty of the Medes will be so great that they will not spare even infant children. People do not commonly harm infants except in cases of extreme barbarity. In short, no consideration will be given to age, as we have said before.

However, we do not read that the Medes exercised such great cruelty. Babylon stood and flourished for a very long time after that calamity; although the seat of the empire was removed from it, the city still retained its name and reputation.

Besides, after dawn the next day, cruelty was committed only against those who bore arms. Although it was the Prophet’s intention to include other judgments of God that awaited the Babylonians (judgments which followed the first calamity much later), he still appropriately describes the barbaric behavior of the nation. He does this so that the Jews might be more fully aware that a just punishment is prepared for Babylon's tyranny.

Nor can it be doubted that, relying on this promise, believers later offered that prayer:

Blessed is he who shall dash thy little ones against the stones.
(Psalms 137:9)