Insert 'select all' choice in a particular level in Taxonomy Term Reference Tree Widget
Drupal have Taxonomy Manager module where all levels and terms have checkbox options and also add, edit, delete, move, functionality here.
Taxonomy Manager:

This module provides a powerful interface for managing taxonomies. A vocabulary gets displayed in a dynamic tree view, where parent terms can be expanded to list their nested child terms or can be collapsed.
For using the Taxonomy Manager you should have JavaScript and automatically load of images enabled in your browser.