show grandchildren when click on their parent

My menu always displayed the parents,children and grandchildren.
I managed to make it stop displaying the grandchildren with setting the depth=1
the problem now is that i would like the grandchildren to be shown when i click on its parent but they are not displayed at any time.

can anybody please give me a tip about what i am doing wrong here?

$html = wp_list_pages("child_of=$parent_id&depth=1&echo=".(!$return)."&title_li=0&sort_column=menu_order");
echo'</ul></div><div class="menufooter"></div>'; }
if($return) {
return $html;
} else {
echo $html; } }
?>