Swift is speaking metaphorically here. He means that the landlords extract so much payment from the peasants, either in money or in crops, or both, that the latter are left with very little. Therefore, the landlords are figuratively "devouring" the parents of the children that he is satirically suggesting should be devoured in fact.
Addendum: No, Persephone, this is not a condemnation of Irish Catholics having too many children, but of the uncaring attitude of the English landlords, and of the English in general, toward the plight of the Irish, who were virtual slaves in their own country.