Make parent page only link to first subpage

class Themeslug_Page_Navigation_Walker extends Walker_Nav_Menu {
    function display_element( $element, &$children_elements, $max_depth, $depth=0, $args, &$output ) {
        $id_field = $this->db_fields['id'];
        if ( !empty( $children_elements[ $element->$id_field ] ) ) {
            $element->url = $children_elements[$element->$id_field][0]->url;
        }
        Walker_Nav_Menu::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
    }
}

This did the trick.