@Heather_Drake Unfortunately you can’t. Sub tasks don’t disappear they always stay under the parent task. And the fact that recurring tasks are just copies of the task when completed your Parent Task, will always have those completed subtasks.
The other issue you may encounter is that when your main task is completed and recurrs all of the Sub tasks will be recreated and uncompleted and if a due date has been set it may well be in the past.
Have a look at this thread with discussion around the use of Sub tasks. To Subtask or not to Subtask
Jason.