Holden hears the attractive little boy singing the song, and it "cheers him up". It also gives him the insight to see that he'd like a job of protecting children from "falling off the cliff" - allegorical for "growing up".
use somebody- kings of leon walking disaster- sum 41 im just a kid- simple plan welcome to my life- simple plan nobodys home- Avril lavigne untitled- simple plan under the bridge- red hot chili...
in catcher in the rye Holden always talks about change ans how he hates it. the ducks symbolize that change is natural, but not always permanent because the ducks come back.