What is the claim in the story Marigolds by Eugenia W. Collier?

There could be many claims (themes) identified in "Marigolds" by Eugenia W. Collier, but one that is stated explicitly is that innocence and compassion cannot co-exist. In order to be truly compassionate toward someone's suffering, a person needs to have personally suffered, too.  
The story is told by an adult first-person narrator—Lizabeth—who is looking back on an act of childish cruelty that took place when she was fourteen. Lizabeth and her younger brother, Joey, are frustrated by circumstances they cannot control: the summer heat, their family's poverty, their mother's absence, the "formlessness of [their] summer days." To alleviate their boredom, they decide to annoy their neighbor, Miss Lottie, because "annoying Miss Lottie was always fun." Miss Lottie has a lot of misfortune in her life—her home is described as being "the most ramshackle" one in an already-destitute town, and she has a "queer-headed" (handicapped) adult son named John Burke—but despite her hardships, Miss Lottie plants marigolds on her property. The marigolds are tangible evidence that Miss Lottie defiantly refuses to give in to her misfortunes.  
As the story develops, Lizabeth and her brother (as well as some other neighborhood children) throw rocks at Miss Lottie's marigolds and damage a few. Later, Lizabeth's frustration spills over into envious violence, and she destroys Miss Lottie's flowers. Instead of appreciating Miss Lottie's efforts to bring a small bit of beauty to an otherwise ugly place, Lizabeth strikes out at the flowers in misplaced rage. The narrator realizes after her rage is spent that she

could not express the things that I knew about Miss Lottie as I stood there awkward and ashamed. The years have put words to the things I knew in that moment, and as I look back upon it, I know that that moment marked the end of innocence. . . In that humiliating moment I looked beyond myself and into the depths of another person. This was the beginning of compassion, and one cannot have both compassion and innocence.

By the end of the story, the narrator has gained hard-earned adult perspective. She still feels a sense of remorse for what she did to Miss Lottie's marigolds, which is evidenced by the last sentence, "And I too have planted marigolds." Younger Lizabeth's innocence was lost as she destroyed the small patch of beauty in Miss Lottie's yard, but this loss of innocence is what gives the narrator the ability to be a compassionate adult.