Add a new NSXMLElement to an existing NSXMLDocument

Question

I have following NSXMLDocument:

<?xml version="1.0" encoding="utf-8" standalone="no"?>
<parent name=“abcd“>
    <content>
        <number>1234</number>
    </content>
</parent>

Now I would like to add another NSXMLElement directly behind the first <number>-entry

<?xml version="1.0" encoding="utf-8" standalone="no"?>
<parent name=“abcd“>
    <content>
        <number>1234</number>
        <number>5678</number>
    </content>
</parent>

Just expected that I have to implement something like that, but it doesn’t work.

NSXMLElement *number = [[NSXMLElement alloc] initWithName:@“number“];
[number setStringValue:@“5678“];

NSXMLNode *parent = [xmlDoc childAtIndex:0];
NSXMLNode *content = [parent childAtIndex:0];

[content insertChild:number atIndex:1];

How can I add the new node/element to the NSXMLDocument?