The youngest member of the Cratchit family, Tiny Tim is a sickly and angelic small boy whose life hinges in the balance of Scrooge's transformation into a better man.
Tiny Tim is one of a long line of perfect, innocent, angelic, completely victimized small children who die in many of Dickens's works. (For others, check out Little Nell from The Old Curiosity Shop, Jo from Bleak House, Smike from Nicholas Nickleby, and Paul Dombey from Dombey and Son. And these are just the ones who end up dying—there's a whole bunch more that merely deeply suffer. It really was a huge thing with Dickens, and we're totally not kidding.)
What all of these child characters have in common is that in their guiltless suffering, they point a super-duper accusing finger at the faulty and generally awful adults around them. In Tiny Tim's case, he is so extra pure and so wildly good that he even gets to say this line:
He hoped the people saw him in the church, because he was a cripple, and it might be pleasant to them to remember upon Christmas Day, who made lame beggars walk, and blind men see. (3.56)
Boy does that ever hammer it home and then some. Not only is he sick just because the Cratchits don't have enough money to cure him (apparently, since as soon as Scrooge gives them some, Tiny Tim gets well again), but he hopes that others can see him as some kind of walking reminder of the doings of Jesus.
Have you ever met a six-year-old? Yeah, this isn't really how they talk. Or what they think about. But what's more is that this super-magical-child approach to figuring out who is to blame for the world's problems sometimes is so overblown that it collapses in on itself.
As Oscar Wilde said of another one of Dickens's dead kids, "one must have a heart of stone to read the death of Little Nell without laughing" (source).