My nine year old really enjoys this show and has watched every episode. I find it a little dark for a kids' show. The topics covered are quite serious and the episodes can be violent.
Each episode focuses on a fictional girl character from some chapter in America's history. She is typically imperiled by something specific to her historical period. The endings of each episode are uplifting, but the road to the ending is usually rocky and in many episodes, girls lost family members. I found some of the episodes upsetting - but things do generally "work out" in the end (at least for the main characters).
Overall, this is a good show to watch together as a family - so potentially upsetting content can be discussed. I recommend it for kids 8 or 9 and up, but it would depend on the individual child because, as I said, some of the material is pretty heavy.