It is an entertaining series that pretends to be Christian. Easy read for 8-12 year olds.
As an adult reading it, there are a lot of plot holes and abrupt changes that don't have a good transition. I have read a lot of books written for 8-12 year olds; the writing in this series is simply poorly done. It reads like a 5 year old telling you a story with a lot of characters who seem like they should be important, but aren't.
Judah, the lion, was a weak attempt to echo C.S. Lewis's Aslan, and was an extraneous character who had zero impact on the storyline. He literally did nothing.
If you are reading this to relate to Christianity, kind of like the Narnia series, you will be disappointed. The authors state that Elyon is God and Justin is Jesus, and then totally misrepresent their characters. Had they not made that comparison, there would be no problem with Elyon and Justin, but since they are representing God, there are huge problems.
Elyon tells the children to follow their hearts. The God of the Bible never said that. He also never said that you have to love yourself in order to love him. Moral relativism creeps into this series in the guise of Christianity:
You have the power within you to fix yourself.
All people are God's light in the world.
You just have to be a good person.
Find who you really are.
Follow your heart.
Don't judge.
Be warned and ready to have worldview conversations with your kids if you decide to let them read the books.