Display child pages of specific page as 2nd nav bar

So I have a site that I am building for a client and basically I am trying to create a 2nd nav bar that pulls all the child pages under his Members page. I have spent a long time trying to figure it out and going through all the topics under the child page tags and cant figure it out. Any ideas? I am very sorry if this has been answered before.
the last bit of code that i used was

<?php $sub_pages = wp_list_pages( 'sort_column=menu_order&depth=3&title_li=&echo=0&child_of=' . $id );?>
<?php if ($sub_pages <> "" ){?>
<p class="post-info">This section has the following sub pages.</p>
<ul><?php echo $sub_pages; ?></ul>
<?php }?>

it just displays the root pages and when I tell it to display a certain page id it doesnt display anything.

Thank you!