William Butler Yeats's "The Stolen Child" is a poem that combines Irish mythology, mystery and romanticism as it pertains to childhood, and in juxtaposition with the modern world. This being said, the main theme that we can gather from the poem is the struggle to maintain the innocence of childhood versus the duty of having to experience of the reality of life. In the end, the faeries will (we assume) protect the child from having to go into the duties of the grown-up life. 

The poet's narrator, presumably a faerie, recites these words to lure the child into the presence of the faeries:

Come away, O human child!To the waters and the wildWith a faery, hand in hand,For the world's more full of weeping than you can understand.

This verse encompasses the main idea: as the faeries call the child, they promise that the child will return to the place where children belong; a place where they can go wild, and roam free. Why bother with the problems of everyday life, when there is a far greater and better solution. Why not protect the child within and leave it be?

Therefore, when the poem reads 

Away with us he's going,
The solemn-eyed -He'll hear no more the lowing
Of the calves on the warm hillside
Or the kettle on the hob
Sing peace into his breast,
Or see the brown mice bob

these verses serve two purposes: to show how the faeries' primary purpose is to bring an innocent child into their realm and away from the real world, and to use romantic language to engage the reader into visuals of a bucolic nature.

This is what the aesthetic style of Pre-Raphaelites like Yeats mainly attempted with their works: to combine the beauty of nature with the reality of daily life. Yeats adds to it the mystery of mythology, and an ample Irish scenery, to create the atmosphere of solemnity and nostalgia that the poem intends to convey.