Why would Chopin have thought it important to include this detail in her brief mention of the children's nurse?

"A quadroon nurse followed them about with a faraway, meditative air."

Is this the mention you're asking about? If so, this comes from the first chapter, which is introductory. Its intent is to give us perspective on the lifestyle of our characters. During this scene we see the relationships and initial actions of the characters; the Pontelliers are busy, preoccupied, and although the nurse isn't important enough to even be named; she too has a preoccupied air about her. The faraway, meditative air, could be said to describe each of the characters...... everyone's main concern seems to be of themselves, and the fact that they each seem to be anything but satisfied with where they are and what their doing.....