“Holden’s problem is that he cannot reconcile the ‘nice’ world of childhood with the ‘phoney’ adult world.” Do you agree?

I think that is part of his issues but there are other problems he is dealing with as well. Holden does hold childhood as sincere and pure. Unfortunately he dismisses the rest of life as phony. I think Holden's cynicism comes from depression and isolation. Holden uses childhood as a refuge that is no longer accessible to him.