What are the developmental aspects of grief and bereavement for the following life stages: 1. Childhood & 2. Adolescence?

What are the developmental stages that are believed to be the most difficult in dealing with death and grieving? Why are those stages more difficult than the others?

Why it is important for mental health workers to know the significance of culture and spirituality within stages of development in the life stages in regard to grief and bereavement?

Peer-reviewed scientific sources are welcomed. I am having difficulty finding sources that are 10 years old or younger.