Holden's fixation is on holding off adulthood/maturity and death. His younger brother Allie has died and Holden's response is deep and thoroughly impacting emotionally and psychologically.
We see this fixation symbolized by the passage in question as Holden imagines himself saving children from death and, by extension, also saving them from the inevitable effects of time. We also see Holden's fixation on maintaining youth in his relationship with his sister and in his continuing relationship with his dead younger brother.
Holden clings to the places and memories from his own youth and things that symbolize youth.
The park evokes his own fond memories of childhood, before his brother Allie's death, and seeing Phoebe circling around in this natural setting seems to bring him a sense of permanency and wholeness.
Allie is a regular fixture in Holden's thoughts and a source of both comfort and crisis for Holden. We can see from Holden's vision that he feels a lack of power but yearns to achieve power over time and over death as these things relate to childhood and youth.