Well, obviously Stacey is one of the main characters since it's about her goodbye. And the other members of the Babysitters Club are likely main characters too, like Kristy, Claudia, etc.
It's about the Babysitters Club and the departure of Stacey. Maybe it involves how the club members react to Stacey leaving, and the changes it brings to the club.
Obviously, Stacey is one of the main characters. Since it's about her mistake, she's at the center of the story. The other members of the Babysitters Club like Kristy, Claudia, Mary Anne, etc. are also likely to be main characters as they are usually involved in each other's stories in the series.
The main plot likely revolves around Stacey making a mistake. In the Babysitters Club series, Stacey often has her own personal and babysitting - related challenges. Maybe she makes a mistake while babysitting, like misjudging a child's needs or getting into a conflict with the other babysitters.
Well, 'The Babysitters Club: Dawn and the Impossible Three' is also worth reading. It shows how the club deals with difficult babysitting situations, and you can see the different personalities of the club members shine through as they work together.