Some of my personal favorites, which my kids also enjoyed at early elementary school age (either read to aloud, or on their own) are:
Jumanji (the movie was based on this)
The Diamond in the Window
The Swing in the Summerhouse
Both by Jane Langton (Kid heros, great adventure, but also a good introduction to classical authors, american philosophers, and existential philosophy)
The Chronicles of Narnia by CS Lewis
Anything by Shel Silverstein
Harry Potter series by JK Rowling (my son started reading these in 5th grade when they were first published and my daughter loved having these read to her until she was old enough to read them herself.)
The Goosebumps series (only slightly scary ghost/monster stories for kids. These were quite the rage when my son was in elementary school. There are a LOT of these!)
Podkayne of Mars
Between Planets
Have Spacesuit, Will Travel
Red Planet
Rocketship Galileo
Farmer in the Sky
Orphans of the Sky
Citizen of the Galaxy
Starship Troopers
Time for the Stars
Tunnel in the Sky
The Star Beast
Starman Jones
The Rolling Stones
Space Cadet
All by Robert A. Heinlein (I loved these books starting in about 4th grade -- they really sparked my imagination!)
The Forgotten Door
Escape to Witch Mountain
Return from Witch Mountain (the Disney movies are based on these two)
The Sword of Aradel
Jagger, the Dog from Elsewhere
The Magic Meadow
Flight to the Lonesome Place
The Incredible Tide
The Mystery of the Sassafras Chair
All by Alexander Key (Great adventure with kid heros!)
Tunnel Through Time by Lester del Ray
The Secret Cave (based on a true story, originally published under the title of 20 and 10) by Claire Bishop
Encyclopedia Brown series (Boy detective agency)by Donald Sobol
Several of these titles were made into movies. It's fun to watch these AFTER reading the book and gives you a great opportunity to discuss how the story might be changed in the movie, why the screenwriter/director might have made these changes, whether or not the kids thought the book was better than the movie, vice versa, or each great on their own merits, etc. It's a great way to introduce your kids to literary analysis in a fun way.
Most of these should be available through the LA public library, which runs a great summer reading program for kids, with incentive prizes. I think most of the branches have story hour for the preschool - 2nd grade set, too. Also check out Borders, and Barnes and Noble. I believe they also run summer reading programs for kids. There are also wonderful short stories published in Boys Life magazine.
Have a fun and literary summer!