I really liked this series as a girl (though I preferred "Dolly", since her story is chronologically correct). I think I read the Hanni und Nanni books between 10 and 13 years old. Though I found their life in boarding school exciting, I never wanted to go one by myself ;)
The stories are appropriate for readers between 9 and 14, depending on the reader's personal development.
They tell the life of the twins Hanni and Nanni when they start a new boarding school when they are 12 years old. Themes are school, French, teachers, horses, midnight parties, friendship, Tennis, Swimming, in general loyalty and helpfulness and staying sensible.