You have to understand that Swift’s “proposal” is part of a more widely expressed dissatisfaction of how the English upper-class rulers were treating the peasants, especially the Irish. In his essay, Swift is exaggerating the terrible price the Irish paid every day to their “owners,” in taxes, in usurpation of crops, in forced labor. So the proposal (hardly modest) is to sell or give Irish babies to the English as food, like a pig or sheep. In actuality, many Irish youngsters were being sacrificed to English greed, in the form of starvation and malnutrition. The details of such an exchange (describing the babies as "plump and tender") only make the analogy that much more poignant and unacceptable.