It is very difficult to find books for young children in the 4-8 range that address LGBT activism.  Instead there are a couple of books that address LGBT issues in general:

Aladdin’s My Princess Boy

Jennifer Bryan’s The Different Dragon

Leslea Newman’s Mommy, Mama and Me

Leslea Newman’s Daddy, Papa and Me

Vanita Oelschlager’s A Tale of Two Mommies

Justin Richardson’s And Tango Makes Three

Ages 10 and up

Marilyn Oliver’s Gay and Lesbian Rights: A Struggle

Teenagers

Linas Alsenas’ Gay America: Struggle for Equality

Betsy Kuhn’s Gay Power!: The Stonewall Riots and the Gay Rights Movement, 1969

Jennifer Smith’s The Gay Rights Movement