Over and over… and over again, the little girl insists that "we are seven." The speaker keeps coming back at her to disagree, but she keeps repeating the same idea in slightly different terms (though the phrase "we are seven" becomes her particular favorite). The repetition is a sign of her strength, stubbornness, and commitment both to her family and to her general life philosophy: the dead never really leave us. This sort of repetition is powerful.
Title: The poem sets the stage for the little girl's repetitions right here in the title, which is just the first of many "we are seven"s in the poem.
Line 15: This is the first reference in the body of the poem to her siblings. The little girl claims that she and her siblings are "seven in all."
Lines 18-24: The girl explains that two of her siblings are at sea, two are at Conway, and two are dead. The fact that two of her siblings are dead doesn't affect her counting; she says "seven are we."
Line 30: Though the speaker clearly doesn't agree with her counting method, the girl is firm: "seven boys and girls are we."
Line 64: The speaker cannot convince her otherwise; after explaining the stories of her siblings' deaths, still the little girl exclaims: "O Master! We are Seven."
Line 69: Even after the speaker gets all worked up and starts yelling excitedly at the girl, she remains resolute. And the poem ends with her declaration: "Nay, we are seven!" The girl gets the (powerful) last word.