Zeekok publishing has a wonderful Music Appreciation Program. It introduces children to seven different composers and includes biographies, a teacher's manual and music CDs. It can be bought individually or as a set. We are using it with some of our children next year. Here are some more thorough reviews if you want more information.
Classics for Kids is a wonderful web site for teaching kids about classical music. The web site features music, videos, lesson plans, and newsletters you can sign up for. It is a wealth of information.
The History of Rock and Roll looks like a great unit study. It is described as "an excellent unit study to pair with your study of American history, from the post World War II era through the civil rights era and beyond. It is virtually impossible to separate the progression of rock and roll music from the rapid changes that took place in American culture from the late 1940s through today.
Music for Homeschoolers will assist parents in designing high quality musical experiences for their children.
I also love the Piano Guys. They put together music videos with classical music while playing in interesting locations. The song Frozen with Vivaldi is below.
Some places to get you started.
Some affiliate links.