In Nurse's Song of Experience, by William Blake, what repetition is used and to what effect?
In the two Blake poems you ask about, no significant repetition occurs within each poem itself. Significant repetition does exist, however, between the two poems.
Both poems feature adult narrators describing children at play. The adults watch over the children: in the first poem as they play on the green and on the hill, in the second poem on the green and on the dale. Both poems deal with the idea of the children coming home after a day of play. In other words, the situation of the children is identical.
The difference of the two poems lies in the emotions of the different speakers in relation to the children playing and coming home. In the first poem, the speaker's
...heart is at rest within my breast,
And everything else is still.
But the second speaker writes:
My face turns green and pale.
Then the speaker berates the children once they come home, telling them that their days are wasted in play and their nights in disguise: "in disguise" may refer to playing along or going along with society by accepting commonly held perspectives and hopes. Perspectives like that a parent should be naturally elated by playing children. But the days wasted in play is self-explanatory. The second speaker finds no joy or contentment in the playing children, as the first speaker does.
The poem demonstrates the idea that the same situation can be perceived in different ways. Blake often deals with perceptions.