.children([selector]) doesn't work?

.children([selector]) doesn't work?

I want to retrieve all of a given class within a jQuery object that already exists but when I give the children() method a selector it never returns anything. If I leave it blank it returns the imediate children but I can't get it to do anything else.

http://api.jquery.com/children/

When I searched for the children function in the jQuery code it looks like it is only desinged to give the imediat children 

children: function(elem){return jQuery.sibling(elem.firstChild);}


What am I missing here?