In Atonement how does McEwan explore the act of writing whilst also reflecting on the experience of growing up?
He does this through the character Briony, who he makes an avid writer and blooming author. He gives her the ability to write, and tells the story through her eyes, using her voracious love of writing from an early age to spur the concept of creating history, over-using the imagination, and the power of a story, especially if it's false.
As Briony grows up, so does her writing. As a girl, she is rather naive, petulant, overly-dramatic and fantastical; this is reflected in her writing. She pens plays that have death and romance, ultimate lessons to be learned, and she cares deeply about what people think of these plays and their enactments of them. However, she realizes quickly how nonsensical those plays are, and how childish, when she sees Cecilia and Robbie at the fountain. She realizes that she has witnessed a real, adult event, and wants to try to capture that. So, she moves on from her childlike fantasies to real, true moments of life in her writing.
It is after the traumatic events and the big, unfortunate mistelling of facts that Briony really grows up. We see her later on, as a nurse, fully aware of her mistake, and using writing as an escape from reality, and also a way to atone for her wrongs. The entire novel itself is a way for her to try to make things right; it reflects her mature, adult understanding of the events, and her ability to assess them with perspective and insight. She writes the novel to apologize for her naivety as a child, and to try to make amends, even though it is too late for all of it. Often, as people age, the long for that atonement, and to make things better, and that is reflected in Briony's writing of the events at a later date.
I hope that those thoughts helped; good luck!