The Songs of Innocence version of "Nurse's Song" presents a utopian vision of what childhood should be like. The poem's speaker is the nurse of the title (a woman in charge of a group of children). Picture the scene: it's getting late, but the nurse's children are having a wonderful time playing in the great outdoors. The nurse, in turn, is happy that they're happy.
The poem's first two lines focus on the sound of the children's shouts and laughter as they echo around the green (that is, the grassy hill they're playing on). The fact that the nurse describes hearing the children rather than seeing them suggests that the kids have wandered off a bit. This, in turn, suggests that the nurse is monitoring the children, but she's not domineering. Under her care, they're free to frolic and enjoy themselves, so long as they're within earshot.
There's no sense of any danger or risk. As such, the nurse says that her heart is "at rest"—that is, calm and serene. She knows the children are happy, safe, and doing what they do best (playing!). The internal rhyme between "rest" and "breast" lends the line a satisfying sound that evokes the nurse's contentment; everything is in its right place.
The rest of the poem will follow the form introduced here: each stanza is a quatrain with an ABCB rhyme scheme. In other words, the second and fourth lines of each stanza rhyme with each other, as with "hill" and "still" here.
The poem's meter, meanwhile, consists mainly of anapests (trisyllabic feet with a stress pattern of da-da-DUM). More specifically, it alternates between lines of anapestic tetrameter (lines of four anapests) and anapestic trimeter (three anapests):
When the voi- | ces of child- | ren are heard | on the green
And laugh- | ing is heard | on the hill,
This is a riff on something called ballad meter. There are occasional variations (as with the iamb, da-DUM, of "And laugh-"), but otherwise the poem's form is pretty predictable. These anapests and steady rhymes lend the poem a carefree, skipping sound that evokes the children's joy.
Finally, notice how both lines 2 and 4 start with "and." This anaphora will appear throughout the poem (in fact, almost every second and fourth line of each stanza starts with "and"). This subtly echoes the polysyndeton found in many translations of the Bible, making this scene sound like a version of Paradise/Eden/heaven. Consider, too, the grammatical function of "and" more generally: it connects and extends phrases/sentences. The repeated use of "and," then, also subtly mirrors the children's desire to extend their playtime outside (and, perhaps, to connect with the environment within which they play).