Want to be able to use one condition which includes or excludes pages based on weather they are children of a specific page?

Add this code to functions.php:

function is_kid( $pid,$includeparent = 0 ) {      // $pid = The ID of the page we're looking for pages underneath
    //$includeparent = 1 to include the parent page, 0 to exclude
    global $post;               // load details about this page
    if ( is_page($pid) )
        return $includeparent;            // we're at the page
    $anc = get_post_ancestors( $post->ID );
    foreach ( $anc as $ancestor ) {
        if( is_page() && $ancestor == $pid ) {
            return true;
        }
    }
    return false;  // we aren't at the page, and the page is not an ancestor
}

then use this in your page template:

if (is_kid($page_id,1)) //1 to include the parent page, 0 to exclude
    {
        code here
    }