The speaker of "Spring and Fall" might be addressing a "young child," but he sure doesn't baby-talk or condescend to this kid. He treats Margaret with gentleness and respect. He seems to understand why she's "grieving" (1) and to respect that. He doesn't tell her that it's stupid to cry over falling leaves. Instead, he seems almost wistful that her innocence and "fresh thoughts" still allow her to feel sad over something that most adults don't even notice anymore.
Still, he remarks that when she's older, she won't mind these things as much. Instead, she'll "know why" (9) she's crying—it will be because she is fully conscious that she, like the leaves, will wither and die one day. This is a bummer of a conversation topic for a young child, but the speaker almost seems to forget that he's talking to a child by the end of the poem. In fact, he might only be imagining the entire thing. Or maybe he's walking with a child in the woods, and starts musing silently in his head about why she's so sad. Or maybe he's actually having a conversation with himself, using poor little Margaret as an excuse to contemplate his own mortality. We could see the speaker involved in any of these scenarios. Which makes most sense to you?