(I'm a smart guy but reading with my parents made a huge difference imo, it is probably the single biggest thing you can do to give your kids a head-start in life. Give your kids the ability to sponge up as much information as possible during those critical years of childhood brain plasticity!)
I've always been a sensitive person (until cruel kids taught me to hide it) and Watership Down and Where The Red Fern Grows got me bad as a kid... but not like lifelong trauma or anything, just baby's first exposure to death.
I actually asked my mom to compile a list of books she could remember, because a friend had kids and was getting to the age where they were starting to read too. Here's what she came up with (note, no particular order of difficulty or maturity here):
My Father’s dragon series
Where the sidewalk ends, Shel Silverstein
A pizza the size of the Sun
Choose your own adventure books
The Great Brain
The Boxcar Children series
Captain Underpants
Roald Dahl books: The BFG, James and the Giant Peach, Matilda, The Twits, Charlie and the Chocolate Factory/The Great Glass Elevator (not as good)
My Side of the Mountain
No More Dead Dogs
The Secret Garden
Bridge to Terabithia (Deals with death, a little older)
The Lion, the witch, and the wardrobe
Across five Aprils (civil war historical fiction, older)
The Broken Blade (Voyageur) & Sequel
Island of the blue dolphins
The witch of blackbird pond
Johnny Tremaine (a bit older)
The sign of the Beaver
Charlotte’s Web
Because of Winn Dixie
Tuck Everlasting (sad at end)
Ruby Holler
The Adventures of Tom Sawyer
Treasure Island
The Pushcart War
Maniac Magee
Rascal
Frindle
Hatchet (Gary Paulsen)
Mr. Popper’s Penguins (young)
Tales of a 4th Grade Nothing (Juy Blume)
Pippi Longstocking
Mrs. Piggle-Wiggle (young)
A Wrinkle in Time
Moccasin Trail
Mrs. Frisby and the Rats of NIMH
From the Mixed Up files of Mrs. Basil E. Frankweiler
The Egypt Game
The Phantom Tollbooth (young)
Holes (little older)
Sideways Stories from Wayside School (young)
The Wonderful Wizard of Oz
Homer Price (easy reading)
The Indian in the Cupboard
Misty of Chincoteague (series)
Aesop’s Fables (young)
The Eye, the Ear, and the Arm
Redwall series
Watership Down
(I'll throw in the Discworld series too for kids that are getting a bit older! It's probably very readable for, say, an advanced 5th grader. I was tearing through all the redwall books and the Golden Compass series at that point and Discworld is probably comparable. Golden Compass wasn't finished then and The Amber Spyglass ended up being kind of a disappointment. I know I was reading Discworld by 7th grade for sure.)
The right there was on of the most instrumental decisions of my life.
By 5th grade I'd read the entire high school literature section. I joined an adult book readers club to better understand what I was reading. Even the adults in my club agreed an adult life is continual situational ignorance, misunderstood tragedy, betrayal by those close to you, and servitude to the powerful. I no longer believe adult life is quite so harsh, but that attitude prepared me far better than my peers. I've spent a good amount of time talking down people ready to quit everything. Thankfully, I can reference some fantastic hopeful passages, and they have not failed such situations yet.