I would use the built in WordPress Menu Manager (Appearance -> Menus) to create my menu structure.
For each of the ‘dummy’ parent pages you can just use a second link to that pages first child page, and rename it to read however you want.
So if your structure is:
-Parent A
-
Child A
-
Child B
-
Child C
And you want to make it so that clicking on Parent A will take people to the Child A page, just insert a link to Child A as your top-level page, and then rename it to read “Parent A”.
Once you have this structure created how you like it, you can use WP_Nav_Menu (http://codex.wordpress.org/Function_Reference/wp_nav_menu) to display the menu in your header.
If on your interior pages you need to show only the child pages of the current section, you can use a plugin I wrote called WP Nav Plus to set your starting depth and only show the children pages (https://mattkeys.me/products/wp-nav-plus/).
I hope this helps.