Supposedly what you already have is that 

1. You have a reference field of the parent task

2. You have a tree/list view of a subtask.

So in your form view, you should have already had something like

<field="parent_id" />
...
<notebook>
<page string="Child Task">
  <div>
    <field name="child_id" />
  </div>
</page>
...

It depends which view navigation method you used. If you use kanban, then you would :

1. Select from a project kanban, which displays

2. Project Tasks Kanban

Include a link and set context to the Project task where, context={ 'search_default_parent_id': active_id }, should gives you the subtasks of a task.