There's no need to force or rush your student's exposure to the darker, more violent aspects of history or literature. Shakespeare is great, but his material is often incredibly bloody. Same with the history of the 20th Century.
There will be time later to cover anything that’s important. It’s better to encourage compassion than to desensitize a student just to meet some vague goal of covering the "right" material.
Susan Wise Bauer recommends skipping, editing, and postponing material whenever necessary with sensitive children...or, when the subject matter is unavoidable, using the opportunity to discuss people’s actions and what they might have done differently.