It's going to take far more than the reading of one book to take in the breadth and scope of Greek and Roman history, but I think you'll find Helene Guerber's writings a fascinating introduction to the main players and events. Helene Guerber was a well-respected scholar of her day and wrote histories of many of the nations. You'll meet her again in a couple of months when you read the story of the Hebrews. The chapters are very short and manageable for young listeners, and although children younger than 8 will likely be able to follow them, there is a lot of violence that was a part of its time that you may not want to introduce to their sensitive natures. I loved how it helped me to put familiar names and events into their proper place and helped me see the overall sweep of their histories. 

From the Libraries of Hope website