Watership Down is certainly not written for kids - although I first read it when I was about 8 or 9 and enjoyed every minute. It has many complexities and is as much comment on humanity as anything else. It's a very moving story and well written. I remember seeing the film which was quite good and at least in part managed to recreate some of the brooding atmosphere of the book.
Another Richard Adams book was also turned into an animation - Plague Dogs. I haven't read the book, but the film was very disturbing and gave me nightmares for ages - I have no idea why it was targeted at children, because it is about dogs that escape from a vivisection laboratory and they go through the most horrific psychological disturbance as a result of the experiments that have been performed on them. Absolutely horrific film. If you want to see something really disturbing, then see this, but only if you have a strong stomach.
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Time is an illusion. Lunchtime doubly so.
Epona of The Laughing Hyenas