What do the children learn from Cal?

To me, the most important lesson the kids learn from
Calpurnia is that they need to treat people well no matter who the people are.  This is
not something that they only learn from her, but it is the most important lesson that
they learn in the book.

I think that they learn this (or at
least Scout does) most effectively when they bring Walter Cunningham home to eat on the
first day of school.  On that day, Calpurnia quickly corrects Scout when she seems to
think she is better than Walter.

By this and by her own
dignity even though she is herself low on the social scale, the kids learn that all
people deserve to be respected.