changing value in custom fields?

<?php
		if($post->post_parent)
			$children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");
		else
			$children = wp_list_pages("title_li=&child_of="7"&echo=0");
		if ($children) { ?>
			<ul>
		<?php echo $children; ?>
		</ul>
	<?php } ?>

The code above works it gets the children on the menu of the specific page say
Main Menu -> Child 1 -> item 1, item 2, item 3

item 1, item 2, item 3 is displayed but if i go to Child 2 -> item 1, item 2, item 3 it wont work anymore cause of the :
child_of=”7″

is there a way to grab the value from custom fields so that it will change?:) thanx!:)