The tone of Little Women is affectionate and sympathetic. The narrator seems quite fond of the March family. Its third-person omniscient perspective allows intimate insight into the girls' thoughts, feelings, and personalities. When the girls have hard times, the narrator always takes their side.
For instance, when Mr. Davis strikes Amy, Marmie pulls her out of school. To modern readers, this might seem like a strong reaction. But the narrator guides readers to an understanding of why they should sympathize with Amy's plight. The narrator's tone takes on a shade of gravity here, but the tonal shift makes sense given that Alcott drew inspiration from fond memories of her own sisters.
To others it might seem a ludicrous or trivial affair, but to her it was a hard experience, for during the twelve years of her life she had been governed by love alone, and a blow of that sort had never touched her before.
Yes, Amy is physically and emotionally delicate. But the ideal feminine woman of Alcott's time was supposed to be delicate. Her shock at Mr. Davis's treatment reveals her expectations of the world's general goodwill. Her comfortable family life that was "governed by love alone" led her to disbelieve that anyone could treat a woman so harshly. Here, the narrator's tone allows readers to understand and sympathize with a character they might otherwise dismiss as too sensitive. The tone stays fairly consistent throughout the story, always acting in service of portraying the March girls in a positive light.