Not sure about not showing to children. I haven't seen Boy in the Striped Pyjamas, but I think they should be given the information as long as it's in a way they can process. They are the up-and-coming generation who will have to deal with this awful world and I think it's important that such vital issues as human cruelty and environmental damage are addressed early on, with plenty of thought-stimulating discussion about solutions - if there are any.
And let's remember that millions of children are actually experiencing cruelty, and witnessing it daily.
As for me, I used to avoid on-screen violence at all costs until my family persuaded me to watch Game of Thrones. It gave me nightmares, but now I'm sort of hardened and can put those images in a 'box'. Still can't cope with anything horrible to animals, tho' - agree about 'Dumbo', and the worst bit of Game of Thrones was when the beautiful Direwolf was put down, out of spite.