Ignorance and Want are children who appear from inside the robes of the Ghost of Christmas Present. Dickens says of them:
They were a boy and girl. Yellow, meagre, ragged, scowling, wolfish; but prostrate, too, in their humility.. . .No change, no degradation, no perversion of humanity, in any grade, through all the mysteries of wonderful creation, has monsters half so horrible and dread.
“Spirit! are they yours?” Scrooge could say no more.
“They are Man's,” said the Spirit, looking down upon them. “And they cling to me, appealing from their fathers. This boy is Ignorance. This girl is Want. Beware them both, and all of their degree; but most of all beware this boy, for on his brow I see that written which is Doom, unless the writing be erased."
With the revelation of these ragged, pitiful children, the Spirit is cautioning not only Scrooge, but all of mankind. He proclaims that they represent Man's worst enemies -- the state of Want (for food, shelter, etc.) that many suffer and live through every day, but most especially, the self-imposed Ignorance of this state of affairs, the Ignorance in which Man chooses to live his life. Man, according the the Spirit (and Dickens ), must wake up and see what is needed by others and the part each individual can play to ease the pain and suffering of his fellows. The Spirit cautions that, unless Man wakes from his self-imposed Ignorance, he will create his own downfall, his own "Doom."
The Ghost then relates these general predictions about all of mankind more specifically to Scrooge when he taunts him with his own words from early in the story, a demonstration of Scrooge's choice to live in Ignorance:
“Have they no refuge or resource?” cried Scrooge.
“Are there no prisons?” said the Spirit, turning on him for the last time with his own words. “Are there no workhouses?”
For more on Ignorance and Want and the social responsibility of mankind as advocated by Dickens, please follow the links below.