Make parent page only link to first subpage

Question

I'd like to automatically make any parent page link to it's first subpage if it has one. Like so:

• Parent 1 (/#subpage-1)
  • Subpage 1 (/#subpage-1)
  • Subpage 2 (/#subpage-2)
  • Subpage 3 (/#subpage-3)
• Parent 2 (/parent-2)
• Parent 3 (/parent-3)

I found some code that finds all the parent links through a complicated nav() walker. From here I'm not sure how to find the first childs url. I assumed it would be

$children_elements[0]->url

but it doesn't seem to be the case.

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 = "Where do I get the child URL?";
        }
        Walker_Nav_Menu::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
    }
}

Answer 1

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.