The babies who died in the great flood of Noah's time were not human.

NOT HUMAN.

They were a genetic mix between humans and demons, like half human and half demon. And demons CANNOT be redeemed, and their offspring cannot be redeemed either. If God had not destroyed all of them, it would mean that no one born in the future would ever be redeemed. The entire world population would not be redeemable. Some of these half-breed offspring were giants. They were known as the "mighty men of old." (Genesis 6)

Noah and his family still had a pure blood line. So they were allowed to live.