Start with high-confidence christian biography books, then narrow by subgenre or adjacent topics.
Prophet With Honor, Kids Edition: the Billy Graham Story
William C. Martin
Gifted Hands, Revised Kids Edition
Gregg Lewis, Deborah Shaw Lewis
Defending the Line
Alex Carpenter
Who Was Mother Teresa?
Jim Gigliotti
Who Was Jesus?
Ellen Morgan
Shaken: Young Reader's Edition
Tim Tebow
The Children's Book of Saints and Angels
Claire Llewellyn
Saint Patrick the Forgiver
Ned Bustard
Polycarp
David Luckman
The Book of Saints and Heroes
Lang
