Fragment URL in navigation item without a custom link

You can modify the href attribute of specific menu items when the menu is generated.

function wpd_convert_menu_items_to_hash( $atts, $item, $args ) {
    if( $somecondition ){
        $atts['href'] = '#';
    }
    return $atts;
}
add_filter( 'nav_menu_link_attributes', 'wpd_convert_menu_items_to_hash', 10, 3 );

In this example, $somecondition is whatever way you’re identifying these menu items. $item gives you access to the menu item object, which contains the ID of the page and the slug, among other things. You can also look at $atts['href'], which will be the default URL value for that page. $args will give you access to the nav menu arguments, so you can target the whole thing to a specific menu.

You could also hook template_redirect and send requests for those pages elsewhere.

function wpd_check_if_page_should_be_visible(){
    if( is_page( 'services' ) ){
        wp_redirect( home_url('/services/some-other-page/') );
    }
}
add_action( 'template_redirect', 'wpd_check_if_page_should_be_visible' );

You could modify the above to check for a value in post meta, so the slug isn’t hardcoded. You could also query for the children and forward to a child page, so the destination wouldn’t have to be dynamic.

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