only display Pages which have children

Do you want to list only the pages with children? I’m not sure to understand well what you want to do 🙁 If I’m right, you’ll have to do something like this with get_pages() and generate your listing manually after:

// First get all pages
$pages = get_pages();  

// Then, get page IDs
if ($pages) 
{
    $pageids = array();
    foreach ($pages as $page) 
    {
        // Test if page has children
        if ( get_pages( 'child_of=" . $page->ID ) ) {
            // Save it if it"s OK
            $pageids[]= $page->ID;
        }
    }

    $args = array(
        'title_li' => 'Pages with at least one child',
        //TODO: add yout display parameters here
    );
    wp_list_pages($args);
}

I think this could work!