If you're a poet, the surest way to take note of time passing is to write about the seasons. (To everything turn, turn, turn… There is a season turn, turn, turn…) In "We Are Seven," the changing of the seasons marks time going by. But the change of the seasons is not a big or flashy symbol in the poem; it's a subtle reminder that death is not a one-time event in the girl's life. She's been coping with death for a long time—several seasons worth of time, in fact. Unlike many other of Wordsworth's poems (check out this baby for an example), "We Are Seven" is not nature-obsessed, but the natural world does creep in in some key moments and have an effect on the way our little girl sees the world.
Line 36: The little girl tells the speaker that her siblings' graves "are green." They have been buried long enough for grass to grow over them. But this grass isn't just a sign of death; green symbolizes growth, life, a new beginning. Maybe this means even a new way for the girl to relate to her brother and sister.
Line 54: Here the girl talks describes how, after Jane's death, when "the grass was dry," she and her brother John would play at Jane's grave. We're thinking that the dry grass is a way for the girl to signify summer, that time when the hot sun dries out the landscape.
Line 57: Time passes between the time of the dry grass and when John dies: "when the grass was white with snow." Many months have passed since Jane's death, and now here's another death. Time keeps on going on, and kids keep dying. Man, this cottage girl has a tragic life.