The original “Bambi”
For some reason I got my hands on George Orwell's "Animal Farm" at a very young age, as it looked like a kid's book.  That's a fairly bleak book, for example when the pigs sell the draft horse to the glue factory, but only the goat (whose only friend is the horse) knows what's going on.Watership Down is another good one, the character of the Black Rabbit of Inle (the Grim Reaper of rabbitkind) was memorable.However I disagree with claims that these kind of works 'traumatize' young readers.  It's just an introduction to life's realities - everyone has to die, nature is beautiful but also violent, and humans can be crueler to each other than any animal ever could. Kids aren't traumatized by reading, or by playing video games - if trauma occurs, it's due to things like war and poverty and neglect and abuse.