Redirect page to default subpage

You can try something like this.
This will check if a page has child pages and if it does then it will redirect the first page in array ordered by menu order.

// get child pages
$child_page = get_pages( "child_of=" . $post->ID . "&sort_column=menu_order" );

if ( $child_page ) {

    // get id of first child page
    $firstchild = $child_page[0];
    wp_redirect( get_permalink( $firstchild->ID ) );

}