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']);