Once a child ticket is closed it disappears into the closed ticket black hole and disassociates itself from the parent and drops off the list of child tickets. We are always going back to all closed tickets to find the ticket that was once associated with the parent to see its history. Please add option display closed child tickets for a parent ticket if the child has been closed. We use this for task lists for projects and if all tasks disappear there is no way to view history of the project.