It’s generally accepted that Cain and Abel were the first children to be born to Adam and Eve.
But I think that’s a misconception! They are the first children mentioned being born in the bible, but I don’t think they were the first.
Genesis 3:16 “Unto the woman he said, I will greatly multiply they sorrow and they conception; in sorrow shalt though bring forth children…”
In my opinion, it seems clear that in order for Eve to understand the implications of “greatly multiply” she had to have given birth to children prior to the fall. Also, when Cain does kill Abel there seems to already be many people on the earth…it doesn’t seem out of the realm of possibility to me that Adam and Eve had produced many children, maybe there had already been generations of children prior to the fall. The bible does mention three separate groups of people in Genesis – The sons of Men, the sons of God, and the Naphtali – who are described as giants. These groups of people had to come from somewhere…although I don’t want to think about who the sons of God were, the internet thought on that seems that they were people bred when angels and people “hooked up”. That’s weird and messes with my mind, so I’ll just be happy to not know who the sons of God are. If anybody has a better answer, I’ll hear you out.
Adam and Eve had children prior to the fall, Cain and Abel are simply the first children to be born after they were removed from the Garden of Eden.