This book goes into rather more detail about O'Keeffe's and Steiglitz's relationship than I am comfortable giving to a grade schooler, but the book is a good biography and includes MANY color photographs of O'Keeffe's work.
I'm a bit troubled that the name of the series is "Portraits of Women Artists For Children" and a main theme of the book seems to be "Sleep with a famous male artist and you'll be successful." Maybe that's just me.
A better choice for children, although much easier, is "Georgia O'Keeffe" by Ray Spangenburg.