The very first winner of the John Newbery Medal in 1922, Hendrik Van Loon's The Story of Mankind (Lexile: 1260; Interest Level: Grades 5-9) gives a comprehensive history of the Western World from the Ancient Egyptians up to the "present" (which, of course, was just the beginning of the 1920s). The edition I read was the one updated in 1939 to briefly include the fact that World War II was brewing. Robert Sullivan and John Merriman have recently completed an 8th update to this classic, published in 2014, bringing the content up-to-date up through the most important events of the past couple of decades including the Cold War, the Internet, and Social Media.
I found it very interesting that the version I read had claimed to do a history of all of mankind, but qualified it as "pertaining to us," as it had a concentration almost entirely of Europe and North America. There was some passing reference to China (and Marco Polo), but no mention of the history of other countries in Asia. Aside from the slaves in Africa and the Egyptian civilization, there was no real coverage of the rest of that continent. I understand that more current revisions have included a chapter on China, but it still seems as though a lot of "mankind" has been left out.
Still, if you are interested in a very intense history, this book is written in a very conversational tone and is definitely aimed at young historians. I do recommend finding the most updated version, which I thought I had, but my audio version was apparently not the most current. (800 pages, in 2014 edition)
One of the reasons I became an elementary school librarian is so I can read children's books.