Foster, Cass, reteller. Shakespeare for Children: The Story of Romeo and Juliet. Scottsdale AZ: Five Star, 1989. ISBN 0-961-9853-X.
The artwork is Disney-like (this is not a compliment), but within this category of books for elementary school this version is best. Foster both summarizes and abridges, so all the prose is his summary but the quotes are from Shakespeare.
However, I do recommend a few of the picturebook prose versions published recently. These make a better, more thoughtful and individual exposure to Shakespeare for readers from 7-12. The art in these books is extraordinarily beautiful.