Do you know what your kids are reading? Are their books filled with fluff, or substance?
How you define "fluff" and "substance" could vary greatly from how I do. To me, fluff is magicians and
wizards, fairies and bratty kids creating humorous situations at the expense of others.
Most children's books have "valued added" or a lesson learned, but in the grand scheme of things, those stories are watery, reconstituted orange juice compared to eating an orange.
The orange would be a book with not only an engaging story, but also lessons in culture, history, and world religions. Yes, world religions.
If you're a Christian, don't be afraid to--under your guidance--allow your children to read about other religions from the point of view of believers in those religions. Better to expose them to it while you have an opportunity to refute it with the Word of God. Just my two cents on the matter. :)
I'm glad my kids enjoy fluff--we all need down time--and I do let them read it. Within limits.
As with everything, balance is key. Including reading material.

We've taken to reading in themes with our most recent theme being Africa and the Middle East. Below, I've listed five books the kids and I have devoured. I attest to their clean content, but I encourage you to read them along with or before your kids. You will NOT be disappointed. Each of these was a can't-put-it-down kind of book, for mom too.
I give all of them and A+ for substance and take-away value. We enjoyed each one, but they're listed here from "least" to "most" favorite.
Tarzan of the Apes, Step Into Classics.

This is a 95 page, illustrated adaptation of the original. It's 2nd-4th grade reading level, but I enjoyed it because I learned the "true" Tarzan story. The Disney version is entertaining, but quite a bit off from the original story. Take-away value wasn't as high for this one, but I try to include a classic in my themes.
Listening for Lions by Gloria Whelan
This is Rachel Sheridan's story, daughter of African missionaries. When the 1918 influenza kills her parents, she's left in the care of wicked neighbors who use her for their own greedy purposes. "Like the lion, she must be patient and strong, awaiting the moment when she can take control of her own fate--and find her way home again at last."--back cover,
Listening for Lions.
Through
Listening for Lions, we learned a great deal about the influenza
epidemic and, of course, Africa--its wildlife and its people.
King of the Wind, the story of the Godolphin Arabian by Marguerite Henry
A Newberry Medal winner,
King of the Wind is
Black Beauty meets
The Black Stallion and is a must-read for all horse-lovers. Based on a true story, it follows the journey of Agba, a Moroccan slave-boy, who is charged with the care of a beloved stallion. Set in Morocco, France, and England, it in no way lacks action.
Grab a box of tissue!
Ali and the Golden Eagle by Wayne Grover

Also based on a true story,
Ali and the Golden Eagle is set in 1977 Saudi Arabia. An American meets a young Arab boy who has never left his canyon village, never seen a car, electricity, or any other modern convenience. But he
is an expert with hawks, and when his new American, cliff-climbing friend brings him a golden eagle, his world opens beyond what either of them could ever have imagined.
It was fantastic and, of all our reads in this theme, this was Seth's favorite.
A Long Walk to Water by Linda Sue Park
It's hard to put into words how this book affected me. Morgan, my 3rd grader, is still talking about it.
It's based on a the life of one of the "Lost Boys" of Sudan
. In 1985, "Salva walks away from his war-torn village. [He is] a refugee, destined to cover Africa on foot, searching for his family and safety." -back cover,
A Long Walk to Water.
This little book will transform the way you think about modern Africa. The headlines on the news will take on new meaning. You'll never open your tap and watch the water flow without praying for those in Africa who literally spend their lives walking from village to murky waterhole and back again, just to provide water for their families. Endless hours, endless days of walking!
A Long Walk to Water is a book
everyone should read, regardless of gender or age.
You can find all these books on Amazon, but for better pricing, check out your local used bookstore, first.