I found a new source for children's free public domain history books which is affiliated with The Baldwin Project - Heritage Books. Heritage History books are divided by civilization and general reading level.
For each civilization Heritage Books has "written an introduction and organized the associated books, character pages, timelines, and historical maps into eras, or historical divisions."
A little about old fashioned story-based history books from Heritage Books, which I think is worth repeating:
"Most of the books prepared for Heritage History take a old-fashioned, or story-based approach to presenting history, rather than an analytical or modern approach. Our books do not attempt to explain the "underlying forces," that influence history, or critique ancient notions of human rights, or focus on contrasts between cultures. Instead they are based simply on stories that have been retold for dozens of generations regarding individual characters, important conflicts, and events of special interest. The stories are connected by simple narrative threads, and are completely shorn of complicated analysis.
This approach to history is not our own invention, but was in fact, the traditional approach to teaching young people history in almost every introductory history book right up until the last half century. The trend toward interpreting history as social science in the Universities dates back to the 19th century, but it was not until the baby-boomer generation that "social studies" and historical criticism replaced traditional narrative history in elementary and secondary schools. Many of our authors were aware of these trends, and opposed them, specifically stating in their introductions the importance of emphasizing the most interesting aspects of history first, and avoiding the
"dull recitation of the textbook. That in the past which a child is led to see is history; all else is weariness and vexation."
Whatever the merits of this new analytical approach may be, it is considerably less interesting to most students than the traditional approach, and modern students are far less knowledgeable about history than their great-grandparents were."