Most sa the  PortalAlias. Unless I am seeing this incorrectly, this is true in general. BUT - the url in the browser does not just look at the PortalAlias by itself, it must run some DNN program that THEN looks at the portalalias.
  If the "http://www.domainname.com/dnn" runs the default.aspx in the root FIRST. What does the "http://www.domainname.com/childportal" run FIRST ? Is it the roots default.aspx ? the subhost.aspx ? or ??

  thanks john. 
  I see my mistake now, I was looking for the child portal default.aspx in the portal/childPortaliD directory - WHAT WAS I THINKING ?  
  The default.aspx page is in a sub directory off of your dnn directory root, NOT in the child portal dir, or even just the portal dir. 
   Also, the name of the subdirectory that holds the default.aspx is the actual name of the portal itself !!