I'm going to plug my own apps here: Bashbored Tasks and Bashbored Tasks for BlackBerry Playbook. It does exactly what you are describing. You can also re-order your tasks in any way you like. You can move a sub-task up and down the tree, or you can re-order the tasks. If you reorder a parent task, all the sub-tasks move with it.
Just search for "Bashbored" in App World. Feel free to PM me if you have any further questions.