@Rumi_fan Welcome to the Chrons.
Even children's books can (and probably should) deal with the most significant themes of humanity. In the Narnia series, Lewis wrote to children with significant themes in mind. I find that these themes resonate with me as an adult. I think that Lewis respected children's comprehension of and desire for significance as basic human traits. Rereading the Narnia books as an adult, I am fascinated at how certain story elements that I thought were merely fanciful children's anecdotes are actually deeply insightful themes that can only be understood with life experiences.
For example, Edmund's betrayal in
The Lion, the Witch, and the Wardrobe is referenced in
The Horse and his Boy. Pondering Rabbadash's treachery, Edmund muses how he once knew a traitor who repented. As a child, I knew the link and felt good because I understood how the anecdote fit. As an adult, I have lived long enough to have made mistakes and hurt people, i.e. I became the villain of the story... and I have apologized and changed my ways.
Another example is how Lucy reacts to meeting Aslan in
Prince Caspian. Already having met Aslan in TLtWatW, Lucy is surprised to find Aslan even larger in size when she herself has grown taller. She remarks, that she assumed that as she grew in physical stature, that Aslan would not seem as big. Aslan assures her that as she grows she will always find him greater in size. I thought this was a neat trick as a child, but as an adult I see this refers not to physical, but spiritual growth.
Lewis wrote in a manner for the story to grow as we also grow intellectually, spiritually, psychologically, and experientially. I have found that as I age (and hopefully gain maturity) that I find even more significance in the books about Narnia.
I could give many more examples.... like Reepicheep's rebuke of Rhince, the use of the term "not a tame lion" from the first book to the last, Diggory's use of the garden gate, Eustace's failing to properly clean his sword, Trumpkin's obedience to Caspian's command to travel to Cair Paravel, Professor Kirke's logic, Jill's rescue of Puzzle, Emeth's appearance in the tent, Aravis' acceptance of Shasta, Jill's failure to daily recite the three instructions, and both the origin and the demise of Narnia. Upon future readings, I hope to find new affirmations of significance.