Get featured image of the post.

I have this code which get all the child post but only up to the first grandchildren. So the output would look like this:
Surgery –> parent
-1. Surgical –>children
—1.1 method1 –>1st grandchildren
—1.2 method2 –>1st grandchildren
-2. Surgical2 –>children
—2.1 method1 –>1st grandchildren
—2.2 method2 –>1st grandchildren
My problem is, how could I get the images(featured image) of those grandchildren only.
Pls. help.

Note: The 1st grandchildren has also childs but i manage to limit it only.

<?php
if(!$post->post_parent){
	$children = get_pages("title_li=&child_of=".$post->ID."&depth=2");

}else{
	$children = get_pages("title_li=&child_of=".$post->post_parent."&echo=0");

	if($post->ancestors){
		$ancestors = end($post->ancestors);
		$children = get_pages("title_li=&child_of=".$ancestors."&echo=0");
		foreach($children as $childs){
		echo $childs;
		}
}
}
?>