Scripture relates that Eve gave birth to Cain and Able.  This doesn't mean that they were the first two.  We don't know how long they lived before the fall and so have no way of knowing about any other children.  Remember Eve's curse of pains in childbearing will increase, implying that there was something from which to increase. Through the genealogical record, God uses only Able, Cain, and Seth to contrast the differences between obedient people and disobedient people. There were many others but He uses these three as illustrations.

God obviously gave instructions about offerings.  Otherwise how would Cain and Able have know about it?  Why was Abel’s offering more pleasing to God than Cain’s offering? Could it be because of the attitude of the giver? Did Cain give his best? Cain brought some of the fruits of the soil. But, (implying a contrast) Abel brought fat portions from the firstborn of the flock. Abel's offering required the killing of an animal demonstrating that he was bringing more than just an offering, he was bringing a sacrifice. Apparently Abel knew that he needed forgiveness of his sins, and that could only come with the blood of an animal sacrifice. However, if Cain was bringing only an offering, that should have been done from the firstfruits, the best of the first of the fruits.
Hebrews 9:22
In fact, the law requires that nearly everything be cleansed with blood, and without the shedding of blood there is no forgiveness.

Could it be that Cain had no sense of sin or need of atonement? Did he think he didn't need to be forgiven.

Gen 4:7
If you do what is right, will you not be accepted? But if you do not do what is right, sin is crouching at your door; it desires to have you, but you must master it."

Just as Satan tempted Eve, he is crouching at our door waiting to tempt us as well.

1 Pet 5:8
Be self-controlled and alert. Your enemy the devil prowls around like a roaring lion looking for someone to devour.

Notice the ages of the people listed in the genealogies. Why did people live so long before the flood? Email for an updated copy of The Biblical View of Creation (aka Deep Sea Diving into Genesis 1) study. It explains all about the protective canopy that allowed people to have longer and healthier lives.