get only sub category ID

As you only want a select form field that has a categories children, then simply go with wp_list_categories()

wp_list_categories( array(
    'child_of' => 'your parent cat ID',
) );

Depending on your use case, you could as well use

• get_the_category() and its get_the_categories filter

  apply_filters( 'get_the_categories', $categories );
  

• get_the_terms() and its filter (the function is called internally by get_the_category():

  apply_filters( 'get_the_terms', $terms, $post->ID, $taxonomy );
  

• get_categories(), pass a special key to the arguments input array (e.g. my-special-list) that you then check in a callback on the filter inside the function and alter the output there. The function calls get_terms() after the filters callbacks get called.

  apply_filters( 'get_categories_taxonomy', $taxonomy, $args );
  

• get_term_children( $term_ID, 'category' )

Edit The main queried object can always be retrieved using get_queried_object(). And there’s a function to extract its ID: get_queried_object_id().