I have reverted back to v 4.6.2 and the child portal is now functioning correctly, so the emergency is over (due to your everlasting recommendations to back up the site and db)
Essentially, last weekend I performed an upgrade from 4.6.2 in several stages, up to 5.1.4, tested on my local dev machine, uploaded it to Windows Server 2003 and everything seemed alright. I never thought to check the child portal much until I started receiving complaints yesterday afternoon and discovered to my horror that the child portal links were simply redirecting to the parent portals login page.
I found some posts regarding this issue but no one seems to have found an answer (http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/107/threadid/321960/scope/posts/Default.aspx)
The one difference is that on the server the site is secure, so maybe the https is screwing things up?
I tried preceding the urls for the child portal with \\childportalname\\ but that didn't really help so I undid that
I tried altering the FrindlyUrls in the host settings to no avail
I thought I fixed it before I went home last night, really without knowing how, because it was working correctly when I left. But today it was all awry again. It seemed an intermittent problem. I have no idea what to make of it. Any suggestions would be more than welcome