Ignorance, the boy, can be interpreted as symbolizing the ignorance that keeps the poor, in Victorian England, poor. Without education, what chance can they have of bettering their situation? If people are ignorant, it is hard for them to know what to do to find a job, to know how to keep a job, and so on. If, however, the poor are given a chance to be educated, and to feel the care of society at large, then they will be much better positioned to help themselves. Want, the girl, can be interpreted as symbolizing the sheer lack of resources and the need experienced by the poor. Although we often associate "want," in the twenty-first century, as a pure desire for something that we do not actually need, the word itself does mean to be needy or to have unmet needs. The Victorians would have understood want to signify these meanings.
However, I do think it is possible to interpret Ignorance in a different way: he could symbolize the ignorance of people like Scrooge who refuse to acknowledge the plight of the poor, who purposely remain ignorant of their needs and pain. Back in Stave 1, for example, when the gentlemen come collecting for the poor, they tell Scrooge that many of the poor would rather die than go to the prisons or workhouses. Scrooge responded, in part, "excuse me—I don't know that," and he claims that it is "not [his] business" to know of other people's problems and lives. The gentlemen tell him that he "might know it," essentially, if he cared to. Scrooge purposely remains ignorant, and this ignorance, if adopted by more and more people who could actually help those less fortunate, will lead to greater suffering for the poor and could spell the "Doom" that the Ghost of Christmas Present warns of.