Violence is a part of life as is generosity and kindness and a whole range of actions and motivators in story. Everything that happens in a story must have relevance to the story, i.e. if you have endless saccharine sweet characters being loving and lovely it will get sickening. So also if you have endless violence and people drowning in pools of blood, it becomes tiresome. So, if someone dies in your story, you must build in consequence, e.g. someone else is sad, or thoughtful, or wonders why it had to happen. Just one opinion.
Messages are the death of an honest literary transaction with children.
After all, everything from their cartoons to their home lives are filled with it. Video games, natch. I have encountered this dilemna myself in the past. What it is, where it comes from and what are the results of it. I asked students to answer these questions, putting the answers on the board for all to see.