Separate and display only particular parent term from a taxonomy vocabulary
I think this is, what you searching for: Taxonomy Term Reference Tree Widget.

Here is a quote about it (from the module's project page):


  ... 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.