Tap the task icon to get to your task list.

Tap the (+) icon to create a new task. 

This task will be your primary task (parent).   Give it a name and all the attributes as fit your needs, make sure to assign it to the sync service you want it to sync to at the top of the task entry and choose a project in the project section then save the task.

Now create another new task.  This task will be a subtask (child). You can create as many subtasks of a primary task as needed.   Give this sub-task a name and include the attributes as fit your needs, also make sure you have assigned it to the same sync service and project as the parent you created.  

Now within that sub-task you will see the option for 'parent'.  Tap this and from the list that appears find the primary task you created.  Then save the task.