How to unregister a menu in a child theme

Hi folks,

I’m writing a child theme of responsive and I want to unregister a couple of the menus.

so I’ve put:

unregister_nav_menu( 'top-menu' );
unregister_nav_menu( 'sub-header-menu' );

in my child themes functions.php, but I think the child themes functions.php is executed before the parent’s – so my unregisterd themes hadn’t been registered and the parent gets to register them afterwards anyway!

Is there a way to use a child theme to unregister menus that are defined in the parent?

Many Thanks

Peace

David