The caste system is presented in the story as an unavoidable feature of Indian social and cultural life. But it's one that people react to in different ways. For her part, Rukmani doesn't want her own children to "marry down"—that is, to get married to someone from a lower caste. There's a good reason for this: Rukmani did the same thing herself. All her sisters were able to marry men from their own caste, the elite Kshatriya caste. But by the time it came for Rukmani to get married, there was no money left to pay for a decent dowry, so she was married off to a man from the lower Vaishya caste.
This causes Rukmani no great sadness and suffering, and she doesn't want her own children to go through the same traumas. The children understand this, but their way of avoiding the heartache of marrying beneath them is not to worry too much about the caste system in the first place. For instance, Rukmani's first two sons, Arjun and Thambi, get jobs in the local tannery, the kind of menial work normally performed by members of the lowly Shudra caste.