The poem's speaker is the nurse of the title (a woman in charge of a group of children). When the poem begins, it's getting late, and the sun is setting. The children in the nurse's care are still playing outside.
Line 1 actually beings with the same phrase that begins the version of "Nurse's Song" from the Innocence section of Blake's collection, placing these two poems in conversation. In the other "Nurse's Song," the nurse can hear children's voices "on the green" and their laughter from the hill. Here, however, that laughter gets replaced with "whisperings [...] in the dale," or the valley.
The sounds of joy thus give way to something stranger and more disquieting. Perhaps the whispers speak to the fact that the nurse is now excluded from the joys of youth (because she is an adult); it's as though the children have their own club that she's not allowed to join. The whispers might also suggest suspicion, secrecy, or even illicit behavior.
It's not even clear if these whispers belong to the children themselves or to some other, perhaps sinister, presence. That is, maybe those whispers represent threats surrounding the children as night falls.
Note, too, how the poem locates this whispering down below in a valley ("dale"), whereas the children from the Innocence version of this poem are up on a hill. This might represent a kind of fall (in the biblical sense), in which the children descend from the heavenly heights of joy to the murkier depths of immorality and/or suffering.
This poem has a similar metrical sound to its predecessor, mainly written using anapests (trisyllabic feet with a da-da-DUM stress pattern) and iambs (da-DUM):
When voi- | ces of child- | ren are heard | on the green,
And whis- | perings | are in | the dale,