I found the Insert Plug-in to Div.
Now, as soon as I select an entry in the parent dropdown list, I get
"Not Found
The requested URL was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request."
By any chance do you have the finished tutorial files main.php and childSelect.php so I could compare with mine and figure out what is different?