Hello! You asked why Petra's emergency is the most significant moment in David's loss of innocence.
Petra is David's younger sister. She is discovered to have the strongest telepathic ability in their group. When Petra is in danger of drowning one day, she sends out a telepathic cry. David senses the urgent and anguished cry for help; he rushes to the river and manages to save Petra. Rosalind also senses Petra's fear and shows up to rescue Petra. Both Rosalind and David realize that Petra's gift is greater than their own and greater even than anyone else they know who possesses the ability.
David realizes his predicament: as his sister, Petra is in grave danger. Their father is Joseph Strorm, the puritanically zealous community leader who sees it as his life work to root out deviants and blasphemies. In fact, Joseph is well known for being the farmer who has purged the most deviations in Waknuk. With recent circumstances bearing out more deviations than ever before (all known mutated crops or livestock have to be purged), David realizes that it won't be long before so-called 'deviants' like Petra and himself are blamed for the loss of crops and livestock. David knows without a shadow of a doubt that his own father would not shy away from executing his sister. His dreams about his own father 'purging' or killing Petra bears out his terrible fears for his and Petra's safety. There are few realizations more devastating than the knowledge that one's father would not hesitate to kill his own children to preserve a perverted vision of a perfect society.
Thanks for the question.