Why adult literature ruins that written for children

Neverwhere ruined Un Lun Dun.  

Stardust ruined any young adult fairy-themed romance ever.  

We Have Always Lived in the Castle ruined Green Angel and any other book with insanity.  

Books written for adults are more interesting because they are often better written.  In addition, the books have more accurate situations because adult writers will not think that they cannot write something that would be inappropriate for children.  Some children’s authors do not know how to explain certain situations because they don’t know what the child knows or does not know.  This leads to scenarios in children’s literature where the violence is too staged, the relationships are too scripted and awkward.  Yes, it tells the story but it leaves the characters a bit flat.  

In We Have Always Lived in the Castle Mary Cat actually poisoned her family because she was mad at them.  Then she went on to do things like nailing books to trees to keep herself safe.  When her cousin came, it was because the book fell leaving her unprotected instead of just a happenstance.  She is clearly insane.  She talks about flying to the moon and living there with her sister-it is always nice on the moon.  In Green Angel, the main character’s parents die but she doesn’t kill them.  Instead of nailing books to trees as protection, she sews spiky things to her jacket to keep herself safe.  We Have Always Lived in the Castle has the space to let a character be truly insane and the writer knows how to do real insanity.  Whereas, in Green Angel the main character, who could easily have been insane and it would have made for a better story, is not.  Her parents were killed in a fire while she was mad at them.  It would have been understandable for her to go insane from fear, guilt, and sorrow.  She does the things an insane person might-adding spiky things to her clothing, gathering rocks for her family members-but the author never makes the leap to true insanity.  

It is this resistance to making situations and characters too real that holds children’s literature back.  There are so many young adult books out there about depression or suicide and other problems that real people face, but even those seem almost unreal.  Sometimes there’s an unrealistically happy ending where anyone who wasn’t nice suddenly sees the error of their ways, or a depressed character is suddenly happy and everything is good.  All bullies end up good in the end, just misunderstood.  People always end up either with their crush or with the right person they hadn’t realized they were meant to be with.  Life is not like this.  

Adult writers know this and they don’t write books that lead to a falsely happy ending.  They have realistic endings even when they are sad or unexpected.  If there is a character who completely changes, it is in a more natural and realistic way.  There are probably some adult books with falsely happy endings and adults who want to read them, but there is a greater potential to present a more honest and realistic telling.  

I know there are books written for children that have more realistic characters and situations, but they often read as an adult trying to relate back to children instead of one human telling another human a story.