Not surprisingly, this is a complex question. It deals with death and Romanticism, two concepts that might be interlinked to an extent, but both of which are extremely difficult to articulate. In reading any poetry, I think that it's important to try to find what you see in the poem and base your answer off of this. With this in mind, I hope you can take what is here as a guide or a springboard for further thought.
Fundamentally, I think that Wordsworth wants a "blurred vision" to result from the conversation about death. The speaker believes that life ends when one is buried. The child does not. This is where the "blurred vision" comes from: The discussion of death is a blurred one. Being a Romantic thinker, Wordsworth likes the idea of children expressing universal truths that "see into the life of things." Wordsworth is the thinker who coined the phrase, "The child is the father of the man." In this light, one sees a collision between Enlightenment values and Romantic ideals regarding the issue of death.
For the literary and intellectual generation that preceded Wordsworth, when life ended, it ceased to exist. When someone physically dies, their lives are over. They are buried, and the matter is closed. This would be the thoughts of the speaker who says that there are only five children in the family with the two that are dead.
For the Romantics, though, the issue of death is a beautiful one because it opens up the unknown, the uncertain, and allows ambiguity and doubt to create thought and passion. It is here where Wordsworth feels the child challenges the conventional wisdom with her belief that the two siblings are in fact in existance. The girl is articulating the idea of a soul or some type of life force that goes beyond what is quantifiable, physical or real. It is here where the Romantics would embrace what the child is saying and try to use it to go against the materialist and empiricist view of reality.
Within this dialogue is where the "blurred vision" takes place. One can either dismiss the child as slightly off her rocker or the speaker for being too cold, but in either case the speaker is left wondering and is challenged by the "blurred vision" that is present. Yet, it is precisely in this lack of definitive answers or clarity where Wordsworth would feel that any search for understanding must lie. Out of this "blurred vision," Wordsworth would argue, a stronger and more coherent understanding emerges.