I will say this, in America our kids learn about violent history very early on.
We learn about the War of Independence around 6-7 years old, where we learned soldiers often lost limbs due to gangrene and that medical care was so horrible they had dirty rusty instruments to saw your leg off.
Then we learn about slavery and slave ships around 7-8, and we're asked questions like how would we feel if we had to stand naked in front of everyone (even a young child can understand that being naked in front of people is pretty embarrassing), and we are shown pictures of whipped slaves to see how horrible it is. That's incredibly horrific if you think about it as an adult. But we still had to write a in our composition notebook about what it would be like as a slave, none of us thought about it deeper than "It would really suck to get whipped and have to work for free, therefore slavery is bad". It transitions to the civil war, where we had to write a letter to President Lincoln as to why he should fight to free slaves and why slavery was bad.
Almost every part of American history is bloody and violent.
The point is, American kids don't get scarred for life because of violent history, neither will British kids. As long as you aren't showing pictures of mass graves to them, it's not going to give them nightmares. You can teach age appropriate lessons on history. You don't need to know the "true horrors" of war to understand that it's bad or even have a general knowledge of what happened.
Concepts like death and war are so far removed from kids that it really isn't going to sink in any further than "that's sad/bad". It doesn't need to. All you need to know about WWI at that age is that secret treaties are stupid, and a bunch of countries fought each other because of it.