You haven't said what your context node is when you're trying to do this comparison. So I'm going to assume it's a <child> element. I'm also going to assume all your <child> elements actually have the same name, instead of all having different names. (If they do all have different names then the designer should be persuaded not to do that.) So your XPath expression would be something like this:
And
here is an article on the various axes available.