What is the central idea of the story?
I tend to to think that one of the central ideas of the story is that it is important to ensure that grudges are not carried between children and their parents. Okeke seems to understand this at the end of the story, a closing with an eerily sad note that indicates his revelation came a bit too late for his own liking. Okeke is the aggressor in terms of the conflict between he and his son. He is the one who made a defiant stand and refused any notion of seeking to bring back together the relationship. It is in the end of the story, with the realization that he is a grandfather, does he recognize the silliness in seeking to deny the relationship he has with his son. Perhaps, it is here where Achebe is making the claim that in situations like these, the older generations must recognize the idea that "marriage is a private affair" and accept the fact that children see marriage in such a light. The ending of the story is one in which time seems to be creeping up on Okeke, to a point where he is fundamentally of losing a relationship that he himself discarded:
The old man at once felt the resolution he had built up over so many years falling in. He was telling himself that he must not give in. He tried to steel his heart against all emotional appeals. It was a reenactment of that other struggle. He leaned against a window and looked out... Okeke was trying hard not to think of his two grandsons. But he knew he was now fighting a losing battle... His mind immediately returned to the children. How could he shut his door against them? By a curious mental process he imagined them standing, sad and forsaken, under the harsh angry weather—shut out from his house.
That night he hardly slept, from remorse—and a vague fear that he might die without making it up to them.
In this ending, one sees that Okeke recognizes fully the errors he made in his relationship with his son and grandchildren. It is in this where the lesson about the choices children make and the need for adults to do what they can to be supportive of these decisions becomes critical.