having trouble with insertChildBefore and insertChildAfter in AS3

var xml:XML = new XML(<rootNode>
    			<head>
    				<meta name="template" content="Default" />
    			</head>
    			<mainSection>
    				<itemList>
    					<item>
    						<video src={this.videoURL}  />
    						<img  src={this.src}></img>
    					</item>
    				</itemList>
    			</mainSection>
    		</rootNode>);

var newNode:XMLList = new XMLList("<item><video src="" + _videoSource + ""></video></item>");

var contentNode:XML = new XML(xml.mainSection.itemList.item);
xml.insertChildBefore(contentNode ,newNode)

var contentNode:XML = xml.mainSection.itemList.item[0];
var parentNode:XML = xml.mainSection.itemList[0];
parentNode.insertChildBefore( contentNode, newNode[0] );