with BEF collapse expand items under their parent How to collapse and expand items under their parent using better exposed filter and Nested Checkboxes ..
What i mean ...that all the child items to be collapsed and they expand when the user click on (+ e.g) for the parent or check the parent box.
I am making a car dealer website and i need that to make an advanced search.
Any help will be appreciated.
I fixed this issue and here is the solution for any one want to do the same, based on this conversion Collapse and Expand according to Hierarchy

add this code to your theme and you can use this module to add Javascript or CSS through the views page or block Code per Views Display 

This code will add a + button beside the every parent, which will expand the checkboxes when clicked

(function ($) {
Drupal.behaviors.betterExposedFiltersExpandCollapseNested = {
attach:function (context, settings) {
  if (!('$befSetupDone' in window)) {
    $('ul.bef-tree-child.bef-tree-depth-1').css('display', 'none');
    var $expander = $('&lt;span class="expander"&gt;+&lt;/span&gt;');
    $('ul.bef-tree-child &gt; li:has(ul) &gt; div.form-item').prepend($expander);
    $('.expander')
      .click(function() {
        $ul = $(this).parents('li:first').find('ul');
        $ul.toggle('fast');
     })
    $befSetupDone = 1;
  }
 }
 }
 }) (jQuery);




and another code which will expand when the parent checkbox checked

(function ($) {
Drupal.behaviors.betterExposedFiltersExpandCollapseNested = {
attach:function (context, settings) {
  // first collapse the children
  $('ul.bef-tree-child.bef-tree-depth-1').css('display', 'none');
  $('.form-checkboxes.bef-select-all-none li').has('ul').once('bef-tree', function () {
    $(this)
      .find('input.form-checkboxes:first')
      .click(function() {
        $ul = $(this).parents('li:first').find('ul');
        if($(this).attr('checked')) {
          $ul.slideDown('fast');
        }
        else {
          $ul.slideUp('fast');
        }
      })
   });
  }
 }
}) (jQuery);


Note: You will have to choose (Nested Checkboxex/Radio Buttons) and check this option in better exposed filter 


  Add select all/none links