To be honest, I'm less sure it shows us specific things about class and more a blend of things about race and class. By that I mean, it matters a lot that both Martha and Percy are Black. This is set in the South, and Helen's parents dwell on the Civil War; we can assume some residual negative attitudes towards race.
That said, Helen is allowed to play wildly with these two and they with her. She's treated more as a thing than a person—they get her so mad she bites her own fingers, after all. I'd say that it is as if her limited senses allow her parents to demote her in social class.