What do the children of “Want” and “Ignorance” metaphorically mean?

Dickens is pretty straightforward with Ignorance and Want from the moment they appear beneath the robe of the Ghost of Christmas Present. In short, Want represents the people of the world who need things. Ignorance represents the people like Scrooge who not only refuse to help those who need things, but turn a blind eye and pretend that they don't see the issue at all, even when it is right in front of them.

The spirit warns Scrooge about both of the children, but says "most of all beware this boy, for on his brow I see that written which is Doom, unless the writing be erased. Deny it!” One of the reasons that the boy is so dangerous is because he represents the manifestation of Scrooge's own words.

In stave 1, Scrooge himself conveys the exact type of ignorance that the boy represents. When two men enter asking for financial assistance for charity, Scrooge inquires about the social institutions set up to "help" the poor, such as the treadmill, the Poor Law, prisons, and workhouses. The men state that most people would rather die than use these programs or places. Scrooge responds, "[if] they would rather die . . . they had better do it, and decrease the surplus population." As harsh as this is, he follows with something even more harsh, as he goes on to say, "[besides]—excuse me—I don’t know that.” It's bad enough that he values life so little; it's even worse that he willfully ignores the issues when he could do so much to alleviate them.

While all of the spirits find ways to make Scrooge reconcile his perspective, Ignorance and Want function to make Scrooge, in addition to Dickens's own London of the 1840s, face the social problems that he shares the burden of. Like some of Dickens's contemporaries, Scrooge shows the exact type of Ignorance that the boy represents, and in doing so contributes to the severe case of Want.