I don't like the things Harriet says and the way she thinks about people, even saying her own mother his dumb. She refers to people as fat as well. This is not proper or kind and not accepted in our household.
good if they're old enough to know right from wrong
If your child is old enough to understand that the point of a book is to observe behavior and decide for oneself whether that behavior is right or wrong, this is a great book. Harriet's behavior (some of which is completely unacceptable -- like sneaking into neighbor's houses) helps the reader understand and empathize with her state of mind and furthers the storyline.