Personally, I don’t think it would make sense if Adam and Eve had children before the Fall, or else there would be a segment of humanity that did not inherit their sin-laced nature, and so would be unjustly impacted by the Fall.
Some have argued that Cain’s statement is evidence that there were other, hostile people on the earth (as one might expect if humans evolved), but I think there is a much simpler explanation (and I wouldn’t even consider myself a young earth creationist). Adam and Eve would have had other children besides Cain and Abel before Abel’s death, since Seth is described as being given in place of Abel (Genesis 4:25), implying that the death of Abel was fairly recent before Seth’s birth. Since Adam is described as being 130 years old at the time of Seth’s birth (Genesis 5:3), it makes perfect sense that he and Eve had many children, grandchildren, etc. by that time. These people most likely lived close together for safety, companionship, and division of labor, so Cain knew that when he and Abel went missing, everyone would know about it. Search parties would be sent out, Abel’s body would be found sooner or later, and people would easily deduce (or at least guess) that Cain had murdered him. Cain would then become known as a danger to others, and as people began to spread out, they would keep their eyes open and kill him to protect themselves (or even go specifically to avenge Abel). Besides all this, Cain had no way of knowing how vast the earth was or how long he would live (his father was still in the prime of life at 130), so encountering someone else could have easily seemed inevitable.