Creating navigation out of specific IDs and their children?

I think there’s couple of ways doing this.

1) register_nav_menu, add your menu in Appearance > Menus to the registered menu location, then let wp_nav_menu handle the markup for you

2) register_nav_menu, add menu in Appearance > Menus, then use wp_get_nav_menu_items to get the menu items and write the html markup yourself.

3) Have an $array of parent page ID’s (hardcoded or get them from an option/post_meta/wherever), do for/foreach/while loop with the array and use get_children inside it. You can either use to loop to simultaneously build the html markup or store the parent and child ID’s to another $array and do the html loop later with it.