William Butler Yeats' "The Cradle Song," is a lovely poem, written with a rhyme scheme of ABABCDCDEFEF, in a total of twelve lines. (Every other line rhymes: so A represents one sound and that sound is found also at the end of the third line; B is a new sound, and that sound is repeated at the end of the fourth line, and so on.) The poem is written as one stanza.
The image Yeats presents is that of someone looking with pleasure into the cradle of an infant. Yeats begins by stating that the "angels" are leaning over the baby's cradle because they are tired of guiding the souls of the dead. Spending time with this baby is a joy for them. The child is so "good," that even God is "laughing in heaven."
The Pleiades are considered an "open star cluster," and are also called the "Seven Sisters," which is probably what the "Sailing Seven" refers to. In this part of the poem, Yeats reports that this cluster of stars is also happy, having caught God's ("His") feelings of delight over the infant.
The Sailing Seven
Are gay with His mood.
The first eight lines all deal with elements of the heavens: angels, God, and the "Seven Sisters." The sense, then, is that this child is perhaps a gift from heaven, so wonderful that she brings elation even to those who already exist in the loftiest regions above the earth.
The speaker goes on to say that he sighs as he kisses the child, acknowledging that he will miss the baby when he/she has grown— the age-old experience of every parent: watching a youngster too quickly leaving infancy and childhood behind.
I sigh that kiss you,
For I must own
That I shall miss you
When you have grown.