People misunderstand common law marriage. Statutory law has replaced common law in most areas of the law. Nowadaws, the individual state has to recognize such marriages; Missouri does not. No matter what this couple does to 'hold themselves out' to the community as a couple, they cannot be legally recognized as married.
A bunch of kids and 13 years, why don't they go down to the Justice of the Peace already?
What if it didn't work out? They don't want to rush into things. < /sarcasm >
If they were in Kansas, they could be married by common law.