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.