You already asked that in the http://forum.processing.org/topic/rss-and-xml-help thread. It is bad form to multiply threads on the same topic (and ignore the given answers), it is better to bump up your topic with new information like the one you give above.

And you get null because, as said in the reference, the children have no content, no textual data. The only child of the root is "channel", which has in turn several children. Among them, there are "item" entries.

The concept of "breakdown" is a bit fuzzy... In general, XML is parsed to get a given information and transform it, eg. to visual elements. So you need a more precise goal.

Let's say you want all titles of the feed. Well, I see there is precisely this example (almost) in the reference:

1. XMLElement xml;
2. 
   
3. void setup() {
4.   size(200, 200);
5.   xml = new XMLElement(this, "http://feeds.bbci.co.uk/news/rss.xml");
6.   println(xml);
7.   XMLElement[] kids = xml.getChildren("channel/item/title");
8.   for (int i=0; i < kids.length; i++) {
9.     String title = kids[i].getContent();
10.     println(title);
11.   }
12. }

If your goal is exploratory, eg. to display each tag name, attribute name and value, text content, you probably need to do a recursive function.
Or just to use a better library (see the thread on JAXB).