$('.'+howDoIgetThis+'_parent').click(function(){
    $('.'+andPutItHere+'_child').toggle();
});

// So that an element with the class 'a_parent' opens 
// 'a_child', and 'b_parent' opens 'b_child' etc.

function listen(parent_id, child_id) {

  // Grab the base element to listen for
  var parent = document.getElementById(parent_id);

  // Listen on that element for the click to open the child.
  parent.onclick = function (child_id) {
     
     // Grab and check the child's style.
     var child = document.getElementById(child_id);
     
     // You could check the .style.height, or .style.visibility
     if (child.style.display === 'none') {
          
          // Show the child element
          // You could use inline, inherit, or auto (CSS properties)
          child.style.display = 'block';
          
     } else {
          
          // Hide the child
          child.style.display = 'none';
          
     }

   }
}

function toggle_display(id){
    jQuery('.'+id+'_child').toggle();
}

'<a href="#' . $label . '_target" id="' . $label . '_parent" onclick="toggle_display(\'' . $label . '\'); click_parent(this);">'

function watch_elms(elms) {
// This function will grab the elements to watch and assign a 
// function to show/hide them when the parent (watched) elmement
// is clicked.

   var i, count = elms.length, return_elms = [];

   for(i = 0; i < count; i++) {

      // Assign the elements to an array so we can watch them.
      return_elms[i] = document.getElementById(elms[i]);

      // Now that we're watching them assign an onclick event
      // handler to them.
      return_elms[i].onclick = function () {

         // Check if the child is visible or not and do the 
         // opposite action (show if hidden and hide if shown).
         if (return_elms[i].firstChild.style.display === 'none') {
          
            // Show the child element
            // You could use inline, inherit, or auto (CSS properties)
            return_elms[i].firstChild.style.display = 'block';
          
         } else {
          
            // Hide the child
            return_elms[i].firstChild.style.display = 'none';
          
         } 

      }

   }

   return return_elms;
}

// We should return the elements to get them on a level where
// you can manage them more (if you keep them in a variable 
// you risk losing them when the function exists and the non-
// ability to edit the properties further.
var watched_elms = watch_elms(['a_parent', 'b_parent']);

$(function(){
    $('a.parent').click(function(e){
        //stop browser from doing default action
        e.preventDefault();
        //get parent class
        var classes = $(this).attr('class');
        var index   = classes.indexOf(' ');
        $('div.child.' + classes.substr(index + 1, (classes.length - index))).toggle();
    });
})

<a href="#" class="parent A">Click to toggle One</a>
    <a href="#" class="parent B">Click to toggle Two</a>
    <a href="#" class="parent C">Click to toggle Three</a>
    <a href="#" class="parent D">Click to toggle Four</a>

    <div class="child A">
        Child One
    </div>
    <div class="child B">
        Child Two
    </div>
    <div class="child C">
        Child Three
    </div>
    <div class="child D">
        Child Four
    </div>

.child { display: none; background: #000; padding: 2em; color: #fff }

$(function(){
    $('.parent a').click(function(e){
        e.preventDefault();
        $('+div.child', this).toggle();
    });
});

<div class="parent">
            <a href="#">Click to open child of A</a>
            <div class="child">
                Child of A
            </div>
        </div>
        <div class="parent">
            <a href="#">Click to open child of B</a>
            <div class="child">
                Child of B
            </div>
        </div>