How is imagery used to support the theme in "We are Seven" by William Wordsworth?

The key theme of this poem by William Wordsworth is the
innocence exemplified in children.  The Romantics valued innocence and noticed that it
was often the innocent view that was actually the most wise.  Wisdom isn't necessarily
learned, and perhaps can't be learned.

In the case of this
poem, the speaker encounters a young eight year old girl.  The imagery that describes
the girl, that "she had a rustic, woodland air, and she was wildly clad" suggests her
innocence and connection with nature (another common theme in Romantic poetry.)  She is
young, and therefore not concerned with appearances yet.  There is a natural goodness
that comes with youth.

The speaker proceeds to ask her how
many siblings she has, and she innocently responds "We are seven."  When he asks for
clarification she explains that two of her siblings are away working at sea, two of the
siblings live in Conway, and two of the siblings are dead and buried in the church
graveyard.  In her innocence she INCLUDES these two siblings because they ARE her
siblings and she spends time with them (at their graves) and she knows right where they
are.  The adult speaker doesn't want to include those two in the total number because he
doesn't think one should count dead people as a part of a total number, but he is
clearly looking at the situation from a different, perhaps adult/practical perspective,
but the little girl's perspective is just as valid and is actually more
true.

She has a reality with these two dead siblings.  She
tells the speaker that "she dwells near them with my mother" at their cottage.  That
they dwell in their graves is of little consequence to their reality for her.  She uses
other images to explain the reality of those two.  She tells the speaker "their graves
are green."  This detail of color imagery makes it more real to the girl.  She mentions
the activities she does when she is "with" them:  knitting, sewing, eating and
playing. The suggestions may seem creepy to the adult, but not to her. Wordsworth uses
imagery of each of the seasons, the dry grass and the white snow, to express that all
year long she is with them.  It isn't that she doesn't care about their deaths, but
their deaths are explained in rather matter-of-fact language and their burial is
described with the word "lie" which suggests sleeping, and not something morbid or even
sad.  The speaker is completely frustrated by the girl's insistence that "we are seven,"
but she is absolutely right.