My kids are 6, 8 and 10 and we are starting Of Courage Undaunted tomorrow! We’ll see how it goes. 
Obviously I’m aiming this book more to the 8 and 10 year olds. I also checked out some fun picture books that are informative for the younger one, but I know the olders will appreciate them too (there is a decent amount of text):
American Slave, American Hero: York of the Lewis and Clark Expedition (Laurence Pringle)
The Great Expedition of Lewis and Clark by Private Reubin Field, Member of the Corps of Discovery (Judith Edwards)
My 10 year old will read Dog of Discovery: A Newfoundland’s Adventures with Lewis and Clark (Laurence Pringle). I looked for Seaman, but my library doesn’t have it…
HTH,
Tanya