I certainly know what is and is not therapeutic for a traumatized child.
Show me some evidence that exposing young children to execution benefits them therapeutically. I don't think you'll find any.
By a conservative estimate, parental homicide affects more than 3,000 U.S. children annually. The Virginia authors cite earlier studies, which posit that bereaved children have to negotiate three tasks:
Accept the reality of what happened.
Find ways to tolerate the pain.
Loosen the affective bonds with the deceased and make the energy available for other relationships
Child Welfare League of America: Childrens Voice: Survivors, Not Victims: Children of Murdered Parents
Note that executing the murderer isn't mentioned.