The first thing to consider is that Leila's character, although is sharing her first ball with kids of her own class, does not really belong to the group. She has never gone to a ball before, she lives away in the country, so she cannot keep acquaintances near. Therefore, she is probably quite far from being the sophisticated girl that her friends appear to be.
Mansfield correlates emotion to the innocence that comes from being unaware of the joys of the outside world. This, combined with the regular wonder of new experiences, are techniques to convey upon the reader the real sensations that Leila goes through.
...how hard it was to be indifferent like the others! She tried not to smile too much; she tried not to care. But every single thing was so new and exciting
Leila would pay attention to every detail around her, from the flowers on the other girls' hair, to the dynamics between one of the Sheridan girls and her brother.
Oh, how marvellous to have a brother..she couldn't have helped crying because she was an only child and no brother had ever said "Twig?" to her.
Upon entering the ball, Leila's young and inexperienced eyes see all that glitters even more shiny and delicate. All that was pretty she sees as the most beautiful thing she had ever seen. In all, nothing could have possibly gone wrong in Leila's eyes.
Dark girls, fair girls were patting their hair, tying ribbons again, tucking handkerchiefs ...smoothing marble-white gloves. And because they were all laughing it seemed to Leila that they were all lovely.
Therefore, the way that Mansfield conveys Leila's emotions is by focalizing the narrative through Leila's new, young, and impressionable eyes. All the innocence that comes with wonder adds to the emotion with which Leila draws a strong sense of empathy from the reader.