In chapter 7, why does Papa want the kids to hear the story about Mr. Morrison on Christmas?

In chapter seven, it is Christmas Eve, and part of the festivities include sharing family stories and reminiscing about the past. While most of the stories are fun, Mr. Morrison tells a different kind of tale. His family was killed on Christmas by night men when he was six years old. A white woman accused neighbors of molesting her, and so the night men burned down their houses and killed the inhabitants with swords. Mr. Morrison only escaped because his mother threw him out of the burning house. She wasn't able to get to his sisters in time, nor save her husband or herself. It's a disturbing story from Mr. Morrison's past, and Mama doesn't think the children should hear it. Papa, however, thinks it's important for the children to listen:

“These are things they need to hear, baby. It’s their history.”

Although grim, Papa believes the children need to have an awareness of the world and know how black people have suffered and been attacked.