 Why is the first part of the story so important?
He ran along feeling sorry for four-year-olds, and even for those who were only four and a half, for they were babies and wouldn't dare try slipping away to the ocean alone. Rather than spoil what was clearly going to be a perfect day, he chose to ignore the fact that he had been forbidden to use time-stopping as a convenience for journeying far from home. He chose to ignore the negative maxim that "small children who stop time without an adult being present, may not live to regret it." The ocean lay at the ready, its stilled waves awaiting his command to resume their tidal sweep. It was time itself that Purnie had stopped, not the world around him.
 It shows the reader a skill that Purnie's been practicing
 It lets the reader know that it's Purnie's birthday (which becomes important later)  It lets the reader see how Purnie interacts with his family  It shows the reader a skill that Purnie's been practicing  It give great detail of the setting (which Purnie has to use later in the story to his advantage)