Problem Statement: On a Kanban board, if you move a story to "Selected for Development" status, its children sub-tasks stay in the backlog status, which doesn't make sense.  Here my reasoning: Sub-tasks are meant to be kept with the story.  So if you plan to complete a story, you should also plan to complete (or reject or move) all the sub-tasks that go with it.  It is bad housekeeping to close a story and not its sub-tasks.  Having said that, this functionality both promotes bad habits and is annoying/error-prone since you have to remember to manually move all the sub-tasks to the "Selected for Development" status in order for them to show up on the kanban board.  

Solution: When a story is moved to "Selected for Development," have sub-tasks auto transition to that state also.   However, don't have sub-tasks auto transition for other story workflow status as the rest of the statuses should be set manually by the user.  In fact, it would be good to disallow closing the parent story if its sub-tasks are not in a done state.  

Considerations:  To solve this, you could just make the backlog view work like a scrum board, where the sub-tasks don't show up and instead they move with the story.  However, I've heard some folks would like Sub-tasks to show up in the backlog, so maybe you can add a toggle to enable/disable sub-tasks in the backlog.