I want my members i.e admin of sub-sites to Login to their

Hi,

I want my members i.e admin of sub-sites to Login to their dashboard "ONLY" through my main website Login page. I wanted to block them using site1.maindomain.com/wp-login.php.

And When they Login from my site they should be directly taken to their dashboard,instead of seeing the screen that I attached,please help me.