Most of my children love to read and they really enjoy reading historical fiction. A favorite historical fiction author around here is G.A. Henty, which has taught my oldest son about wars and time periods most people have no clue about. Many of Henty’s books are available for free download in the kindle version or paperbacks are available for under $10.
We have found that history is much easier grasped when read in the form of a story and not just learning dry facts. (I’m in the process of learning history that I never learned in public school.) Heritage History is a free source for time lines, maps and stories about the the characters of history, even those you’ve never heard of. The history is divided up into time periods and also into civilizations so you can go to the section about Rome and read the time line of Rome and the stories of Rome.
Warning though some history is of course violent and there are portions that are not suitable for young children. I would never just turn the younger children loose on the website. Since most of the actual books on the site are in the public domain, if you desire you can search for the title somewhere like Project Gutenberg to download and read offline.