You don't want end users messing around in a text area with XML. XML has a strict form and the users are certain to screw it up.
I'd suggest using the tree widget from the
Struts jQuery plugin. You can see a demo
here, click on "Widgets" then "Tree". You will have to add some controls to add/edit/delete nodes, but this will help you preserve the integrity of the XML while giving the user an easy-to-use interface to manipulate it.