Most of this will go over your 3 yr. old’s head, but your 6 yr. old can absorb the basics.
As Good as Anybody: Martin Luther King and Abraham Joshua Heschel’s Amazing March Toward Freedom:
http://www.amazon.com/As-Good-Anybody-Abraham-Heschels/dp/0375833358/ref=tmm_hrd_title_0?ie=UTF8&qid=1389924046&sr=8-1
I Have a Dream (Book & CD) : http://www.amazon.com/I-Have-Dream-Book-CD/dp/0375858873/ref=pd_sim_b_6
There are some books by his sister, Christine King Farris.
The Cart That Carried Martin – Eve Bunting (great author with beautiful illustrations)
Heroes in Black History: True Stories from the Lives of Christian Heroes – Dave and Neta JAckson
A Picture Book of Martin Luther King, Jr. (Picture Book Biography) (Picture Book Biographies) – David Adler
I’d suggest some poetry from Langston Hughes and Phyllis Wheatley and introducing your oldest to King’s forebearers (of which there are good, young people bios available from Russell Freedman, Cheryl Harness, others) such as:
Frederick Douglass
Langston Hughes
Phyllis Wheatley
Booker T. Washington
G. W. Carver
Harriet Tubman
Benjamin Bannekar
HTH