Вот в таком массиве будут храниться все елементы
db[-1][0] = "0"; // Где [-1] это ParentID, а [0] ID елемента
db[-1][1] = "1";

db[0][2] = "1-st child of 0";
db[0][3] = "2-st child of 0";

db[1][4] = "1-st child of 1";
db[1][5] = "2-nd child of 1";
db[1][6] = "3-rd child of 1";

<option>
    <select value="" selected="selected"></select>
    <select value="0">0</select>
    <select value="1">1</select>
</option>

<option>
    <select value="" selected="selected"></select>
    <select value="0">0</select>
    <select value="1">1</select>
</option>
<option>
    <select value="" selected="selected"></select>
    <select value="4">1-st child of 1</select>
    <select value="5">2-nd child of 1</select>
    <select value="6">3-rd child of 1</select>
</option>

	<script language="JavaScript" type="text/JavaScript">
function Show_Hide(objId, hBlock)  
 	{
  	var obj=document.getElementById(objId);
      	if(obj.offsetHeight <= 0) {flag=true;} 
 	if(obj.offsetHeight >= hBlock ) {flag=false;}
  	  	if (flag)  			  
                               {  
                             obj.style.height=obj.offsetHeight+10+'px'; 
 			 t=setTimeout(function(){Show_Hide(objId, hBlock)},3); 
       			if (obj.offsetHeight>=hBlock) {clearTimeout(t);}
  			}   				
else  
     {obj.style.height=obj.offsetHeight-10+'px';
       t=setTimeout(function(){Show_Hide(objId, hBlock)},3);
	if (obj.offsetHeight<=0) {clearTimeout(t); }
   }  		  	
}      	</script>

<textarea>большой_текст</textarea>