The speaker of "Spring and Fall" explains that the "name" of sorrow doesn't matter. Everyone is really just sad about the same thing: death. So, whether or not you're able to articulate the reason for being unhappy isn't important. Maybe you'll learn how to express your emotions better when you get older, maybe not. Either way, the idea of expressing yourself effectively is an interesting theme in a poem that uses such unusual grammar and made-up words.
By using made-up words like "wanwood" and "unleaving," Hopkins emphasizes a childlike mode of self-expression that is emotionally effective—though not always grounded in reason or traditional rules. Fantabulerific!
The speaker suggests that Margaret will be able to articulate the reason for her sorrow more effectively when she's older, but provides little evidence to support this. In fact, the speaker's own difficulty in expressing exactly what the sources of his sorrow are suggests that adults can have as much trouble explaining themselves as children. Kettle, meet pot.