"Spring and Fall" is dedicated "to a young child," so you can bet that innocence will be a major theme. Margaret, the little kid to whom the poem is addressed, begins the poem by innocently "grieving" over the falling of the leaves in the forest in the autumn. But as the poem progresses, the speaker suggests that her grief is really over a dawning realization: death is inevitable—for all of us. Dum DUM DUM!
Silver lining time: Learning about death doesn't necessarily have to constitute a loss of innocence—it would depend on how the child learned about death.
It's appropriate that the poem takes place in the fall, since it's about Margaret's "fall" from innocence into knowledge. (See what Hopkins did there?)