So, I started with authors that I knew of. Ingri d'Aulaire was one, and she had a biographical book on Washington and one on Lincoln, but I was only able to get the Abraham Lincoln book from the library... the other was checked out or something. Ingri and Edgar Parin d'Aulaire have biographies of Lincoln, Washington, Benjamin Franklin, Buffalo Bill, Columbus, Leif the Lucky and Pochahontas. The Lincoln biography was winner of the 1939 Caldecott Medal. It's about 55 pages or so, with quite a bit of text on the pages, but great pictures as well. (Becky, I am sure the girls could read it on their own, but their curiosity for information would surely be satisfied!!)
The activity books are called "Abraham Lincoln for kids: His Life and Times with 21 Activities." The Washington book is called the same, but with a different author. These are great books, (although I haven't read all of the text, mostly just skimmed the activities at these point. The activities are geared more towards the girls' age, but Jonah and Naiah would be right on board, I am sure!! )
I got a "rookie biography" on Abraham Lincoln by Carol Greene which is about 45 pages. Reading wise, it might be too easy for the girls, but information wise, it would be just right. There is a lot of information!!
And finally, I got a "Jr. Graphic Biography" on George Washington by Dan Abnett, which I think Jonah might be interested in... It's written almost like a comic book (which I do not understand why boys enjoy them so, but hey, I'm all for it!) So, the text was in short bursts of important information with comic-like pictures. Example: "On March 5, 1770, a group of Americans rioted. The British troops fired on the crowd. They killed five people and hurt six more. This became known as the Boston Massacre."
Anyway, hope this helps. And I would be glad to hear any info. on what books or biographies anyone else wants to recommend. =)