Menu order of parent menu from perspective of child menu

I have developed a theme which has a parent menu and a sub-menu. It looks something like:

Main menu              ==> category | price | depot

sub menu for category  ==> tag| name | date (show/hide)
sub menu for price     ==> currency | date (show/hide)
etc...

I am using a redirect template that redirects automatically to first tab of child menu when i click on parent menu. Ie if I open :

http://host/category 

it will redirect to:

http://host/category/tag

Question

I would like to get the parent's menu_order from child menu. ie , when loaded:

http://host/category/tag

I want to get the order (0,1,2...) of the parent ie "category" tab. How is this done in WP?