The best way to set up a login form on multiple pages, is to have the login form post to the login page. This way you only need to manage one Authenticate User server behavior.
It's tough to tell what the problem is with out seeing the code, but it sounds to me like the Authenticate user server behavior on the child .dwt page is not configured to store all of the same session variables as the other template is. If you take the Authenticate User Server behavior out of the template and have the login form post to the login page, you will only have to manage the login for the entire site in one place.
