On getchild function, when no matches are found ldtp raises a generic exception:

if not matches:
  raise LdtpServerException('Could not find a child.')

It would be more useful to show a message saying what we were looking for. Something like:
'Couldn't find child with name "%s", role "%s" and parent "%s"' % (child_name, role, parent)

Well, I'm not really used to the syntax of string formatting, but that's the idea.

Thanks,
pura vida.