In Genesis 1:28 Scripture tells us that God commanded Adam and Eve to “Be fruitful and increase in number”, but we don’t read anything about their children until chapter 4, after the fall. Did God not consider their children important enough to write about or was Cain in fact their first child? If that is the case can we speculate that the fall occurred almost immediately after Eve was created?
I’ve heard a number of opinions, but we have no way of determining the span of time between Adam and Eve’s creation and their fall into sin. There also appears to have been some span of time between Adam’s creation and Eve’s, and they both were sufficiently familiar with life in the garden to have an informed conversation with the serpent about God’s expectations of them.
It’s pretty clear that Cain was their first child and was born after the fall. Had that not been the case, they would have had unfallen children which would have created huge problems. For example, the unfallen ones could have remained in the Garden after the fall and theoretically could have redeemed their parents. Satan’s plan to cause the fall of the entire human race would have been much more complex.