Ignorance and Want, the children of humankind, cling to the Ghost of Christmas Present because, in Scrooge's (and Dickens's present), they are children, young, a new kind of social problem.  To be sure, ignorance and want have always existed, but they exist in a new way during the Victorian era, a time characterized by an extreme disparity between the haves and the have nots.  People who were poor were excruciatingly so, and many of the people with money -- people like Scrooge -- choose to remain ignorant of the problems caused by this extreme want.  Thus, ignorance and want were the two social evils that Dickens believed posed the biggest threat to society. 

In this era, then, a particular kind of ignorance and a particular kind of want were in their infancy, and they would only grow to become more pernicious if not dealt with early on.  Therefore, they are children who cling to the Ghost of Christmas Present because they are relatively small problems in the present; however, they are problems that will grow more significant that longer they are ignored.