Who did Adam & Eve's children marry?

Dear Rev. Know it all,
I have a question from the 4th grade class that I teach. “Since God created Adam and Eve and there was no one else on earth, wouldn't their children have to have married each other?  (Insert general grumbling, 'yuck,' 'gross!', etc. from the students.)”  Whom did Adam and Eve's children marry?"  
And if you wouldn't mind including, because I think it will also be asked, "By whom were they married?"
I am extremely grateful for your help. 
Miss Inge Link

Dear Inge, 
I must be running out of material. I am going to quote something verbatim from last weeks column: 
 I love the story that Corrie ten Boom tells in her magnificent book, “The Hiding Place.” When she was a little girl, perhaps 4or 5, her father would often take her with him when he went to the big city to buy supplies for his watch repair business. One day, as the train rolled through the Dutch countryside, little Corrie, having heard older girls talking on the playground at school, asked her father, “Papa, what is sex?” Casper ten Boom, looked at little Corrie and silently turned to stare out the window again, leaving his daughter’s question unanswered. When the train pulled into the station, Casper asked Corrie to pick up his tool bag. She did her best  but couldn’t budge the bag. Casper then said to her, “Corrie, there are some things too heavy for you to carry right now. When you are old enough, I will tell you what sex is. Til then, trust me.”  (In telling this story to your children perhaps you can substitute the word “robbery” for “sex.” I don’t want to risk a law suit. It’s odd that your students go home to watch prime time pornography masquerading as children’s programming but if you so much as mention the wrong thing some parent will haul you into court. Oh well. Where was I?)
I don’t know how God worked the whole situation out. Perhaps He made wives for the sons of Adam and Eve the same way he made Adam. Out of clay. Sort of like Gumby. Maybe He himself did the weddings. After all He walked in the garden, the Bible says.  Certainly He had the authority to officiate at weddings.  The point is that I don’t know. I don’t need to know.
The Bible isn’t a history textbook, though it has history in it. It is God’s commentary on the nature of humanity. I’ve said it before and will say it again. The Bible, especially the first chapters of the Bible, are God’s view of real events. He sees them in a fuller and more meaningful way than we can.  Perhaps Adam and Eve were a couple of cave persons. God saw more. Perhaps the ark was a flat boat on a flooded Mesopotamian plain. God saw more. Perhaps the Tower of Babel was just a three-story mud hut where a family had a really bad fight. God saw more. Perhaps Abraham was just a greasy desert wanderer. God saw more. Perhaps you and I and your little students are just short lived blips in the cosmic scheme of things. God sees more. 
That’s the point. We look at things and pretend we can take in the whole reality We can’t That’s why God gives an interpretation of these great realities and then says, “Trust me.” He tells us just enough to get us to heaven,. We want to pick apart the text in a way hides its meaning rather than reveals its meaning. I’m not saying that scholarship is a bad thing. The more we understand about the language and the context of the Scriptures, the more fully we will be able to hear what the Holy Spirit is telling us. However, a lot of so called scholarship assumes that if you can’t see it or touch it, it isn’t real and has no meaning. That was the very sin of Adam and Eve, the original sin.
Read the text before you pull it apart. Eve looked at the fruit of the tree and saw that it was good for food and for the gaining of knowledge. In other words she believed that she would be God’s equal and not have to be His child. She would no longer have to trust Him. So it is with us old folks, and believe me I have met some very old fourth graders, real cynics.  Mary, our Blessed Mother, when confronted with an impossibility, “Behold the Holy Spirit will overshadow you” said. “Okay. I’ll trust God. Whatever He wants.” The new Eve trusted. The old Eve connived. What the text says is so much more important than what the text leaves out. When the devil gets us to look at what the text doesn’t say, he manages to keep us from hearing what the text says and says so beautifully.
I am reminded of W.C. Fields, the great comedian. A friend came to visit it him as he lay dying. He found Fields reading the Bible. He said “I thought you didn’t believe any of the that stuff h. Why are you reading the Bible?” Fields responded, “I’m looking for loopholes...” Sometimes, when we try to find out what the Bible doesn’t say instead of hearing what it says, we are doing exactly the same thing, looking for loopholes.
As I have said before, some people have the souls of poets, others have the souls of appliance repairmen. When a poet says “her lips were like roses, her eyes like flame,” the literalist will say “How did she keep from burning her eyebrows? And did she have thorns to go with roses? That’s gotta hurt.”  
So tell your little cynics this: God only tells us the things in the Bible that we need to know. When science describes things one way and the Bible describes them another way, God is being a little bit poetic in order to help us understand, but He’s telling us more about the story than even scientists can.
I am reminded of one more story. St Augustine was walking by the shore trying to understand the mysteries of God, He saw a little boy digging a  hole in the sand. The little boy would dig the whole, run to the water with a clay pot and pour the water into the hole, which would then collapse. He repeated the process over and over and over until finally St, Augustine asked him what he was trying to accomplish. The boy said, “ I am trying to put the whole ocean into my hole in the sand.” Augustine laughed and said, “you’ll never fit the whole sea into that tiny hole. It’s too small and it collapses constantly. The little boy said, “And Augustine, you will fit the greatness of God into your little mind,” and then the little boy vanished from sight.
Yours,
Rev. Know-it-all