First and foremost, as satire, it deals with a variety of social issues. The underlying purpose behind the essay is to criticize the British government's attitudes and treatment toward the Irish in the late 18th century. He creates sympathy for the destitute conditions fo the Irish by adopting a tone which inspires disgust in his audience. As one reads, he/she identifies with the Irish, not the British landlords. He continually represents these landlords as vile creatures, describing how they have eaten up the parents and are now moving on to the Irish children. Thus, his mocking suggestion to eat the children serves as a metaphor of how the parents spirits, money, etc. have already been devoured.
He also mocks the can-do spirit of the time, when people offered wildly illogical solutions to reasonable problems. Thus his true solutions are hidden in a list which he derides, although they are the truly sensible answers to the situation. Coupled with this, he attacks the contemporary prejudices, such as the attitudes toward Catholics and Americans.
Finally, one must understand the mercantile system of economics under which Swift was living. No child was too young to work, nor was anyone considered unfit for labor. Therefore, if you did not work, you were labelled as lazy and a burden on society.