Assign to node all child terms if parent term selected
You can use the Taxonomy Term Reference Tree Widget module 


  This module provides an expandable tree widget for the Taxonomy Term
  Reference field in Drupal 7. This widget is intended to serve as a
  replacement for Drupal's core Taxonomy Term Reference widget, which is
  a flat list of radio buttons or checkboxes and not necessarily fit for
  medium to large taxonomy trees.


How to use this module


Enable the Term Reference Tree module.
Go to the Manage Fields tab of any fieldable entity (such as a content type, taxonomy term, or user.
Add a new field with a type of Term Reference.
In the widget select box, select Term reference tree.


Note that you can also change any Term Reference field to use the Term Reference Tree widget, by editing that field and selecting Term Reference Tree on that field's Widget Type tab.