There was -- at that time -- children who were born inclined to great wickedness. Abraham was told his offspring would not enter the Land of Promise until the "iniquity" of the Amorites was full.
Genesis 15:16 But in the fourth generation they shall come hither again: for the iniquity of the Amorites is not yet full.
As long as the iniquity is incomplete, there is a chance for repentance. God often defers judgment then until the wickedness is so great, there is no hope for change.
We see something similar in Sodom. For some odd reason, the KJV changes the order of words -- so let me cite Young's Literal.
YLT Genesis 19:4 Before they lie down, the men of the city -- men of Sodom -- have come round about against the house, from young even unto aged, all the people from the extremity;
Why young first? They were the instigators. The adults were wicked enough; but their children were even worse. It was God's plan, of course, that every nation and tribe be represented in Heaven; so Lot's two daughters played a key role -- and both became ancestors of Jesus eventually.
The intense evil present in the world then has been largely dealt with; but back then, I believe some parents were so evil, God did not permit children to be born to them. Their offspring were demonic. If I'm right, thank goodness that evil was wiped out.
These were the wicked ones David said were corrupt from birth.
Psalm 58:3 The wicked are estranged from the womb: they go astray as soon as they be born, speaking lies.
I don't think of them as human to tell the truth. They looked like humans; but they had nothing of the image and likeness of God about them. They were like the beasts of the earth but worse since they were demonic.
They weren't completely wiped out though. They attempted to marry into the Aaronic priestly line -- they did in fact -- Joshua the High Priest had done nothing when his sons married pagan women. The serpent's seed was trying to infiltrate the line of the High Priests. That is what got him in trouble with Satan, the Jews say. He repented and had his sons divorce those pagans, so he was given clean garments again by God.
I think too that the seed of the serpent had intermarried with the priestly class over time -- which was the reason Jesus said they were children of the serpent, not of Abraham. The whole Temple system had been undermined. I think many of the serpent's children got wiped out in 70 AD.
It is a hard concept to fathom of how this could be; but I don't think we have to worry too much about it today, thanks to David and others like him. Don't forget that David eliminated much evil, and then his son reigned in peace. Judah is like that -- the lion that rages until "shiloh" or "peace" comes.
Genesis 49:10 The sceptre shall not depart from Judah, nor a lawgiver from between his feet, until Shiloh come; and unto him shall the gathering of the people be.
In the New Testament, we see it being said the Lion of Judah will open the Seals, but the lion doesn't. The lamb does. The lion has become the lamb.
Revelation 5:5 And one of the elders saith unto me, Weep not: behold, the Lion of the tribe of Judah, the Root of David, hath prevailed to open the book, and to loose the seven seals thereof.
6 And I beheld, and, lo, in the midst of the throne and of the four beasts, and in the midst of the elders, stood a Lamb as it had been slain, having seven horns and seven eyes, which are the seven Spirits of God sent forth into all the earth.
7 And he came and took the book out of the right hand of him that sat upon the throne.