I don't remember Confessions of an Only Child, and it's certainly not necessary to have read that one first to enjoy this book.
We have our classic Norma Klein setting of an apartment building in New York City. Antonia is ten with a baby brother and she walks her neighbors' dogs. She has a friend who's a boy and a best friend, Libby, who doesn't want to let her in her "tomboy club" because she's not boyish enough. Her dad is the one taking care of the baby while her mother works.
A major story line is that Toe starts her period very early, like her mother. It's not written in as much detail as a Judy Blume story line and is not the main focus. The girls do have those great, frank conversations about getting married and whether or not to have babies that seem both very 1970s, and also very ten (ex: they don't use the word sex.)
Another great addition to my Norma Klein collection.