The story of marlin card, who was born into poverty as a child and has lived his life in and out of prison, demonstrates what, according to card himself?

I believe the answer is: the importance of quality time spent between parents and children, which is so difficult for the poor to find.

The story indicates that the parents in the poor family would most likely to spent most of their days doing long hours in their jobs to fulfill the family's ends meet.
This would make them do not possess enough time to properly pay attention to their children.