By the second stanza, we see the speaker using the title (and question) as a refrain. The repetition really adds to the singsong vibe of this nursery rhyme, making it difficult to forget.
If a child were listening to this, he or she might even chime in at this point and sing along. Mission accomplished.
Line 6 also sounds the same as line 2, only the speaker has reversed the "I" and "you." Why would she do that? Well, nursery rhymes often play with words, sometimes reversing the order of words or repeating key phrases like you see here. Check out these famous rhymes for a better idea of how that works.
Since we're dealing with pretty much the same lines as the first stanza, we can assume we're hearing the same headless iambic trimeter pattern as well. The speaker isn't throwing in any crazy experimental wrenches. She's keeping things nice and simple for the kids (and us).
But when the trees bow down their heads, The wind is passing by.
Just like the first stanza, the second stanza ends with a reaffirmation that although we can't "see" the wind, we know it's there because of the way the trees move.
So the trees are the only evidence we have in this poem that the wind does in fact exist. When they "bow down their heads" we know that wind is passing by again.
Notice we have some more personification in line 7, just like the first stanza, that gives the trees heads. So the speaker isn't just using familiar meters and a refrain to maintain the poem's form. She's also using devices like personification and repeated imagery to keep us imagining those trees in a way that's more animated as if they're people, too.
We might also imagine an invisible understanding that the trees share with the wind. They "bow down their heads," which is a sign of supplication or reverence. So the trees appear to respect the wind and give way to its presence rather than fight against it.
Again we feel a sense of the speaker describing nature in the same simple and effortless way that it exists. There's no need for elaborate descriptions or big meanings here. Nature gets along just fine in its "passing" way.
And just like before, we see the same meter we saw in lines 3-4: iambic tetrameter followed by iambic trimeter. We also have another perfect end rhyme between "I" in line 6 and "by" in line 8. Don't forget to check out "Form and Meter" for more.
By the end we really feel the speaker's message here that's simple, lyrical, and yet purposeful in its own way. We may not be able to "see" nature in the forms we'd like to, but we nevertheless feel and know its presence, which is evidence enough that nature is alive and well.