show “grandchild” in itheme

I’m using the itheme, it has a good menusystem allready “baked in”. But I was wondering how I could show the “grandchildren” in the menusystem. Or actually at the moment I would pretty much settle for anywhere :).
I’ve tried using this code:
<div id=”subnav”>

<?php if(wp_list_pages(“child_of=”.$post->ID.”&echo=0″)) { ?>

<?php wp_list_pages(“title_li=&child_of=”.$post->ID.”&sort_column=menu_order&show_date=modified&date_format=$date_format”);?>

<?php } ?>

</div><!– /sub nav –>
<?php } } ?>
But then it is shown when you open a parent page (G.c = Grandchild:
Parent 1 Parnet 2
Childpage 1.1 Childpage 1.2 Childpage 1.3
G.c 1.1A G.c 1.1B G.c 1.2A G.c 1.2B G.c 1.2.C G.c 1.3.A etc.
And that dont look nice.
When you then open a child page it looks like this:
Parent 1
G.c 1.1A G.c 1.1.B
This looks nice and would like to keep this part.
(Hope you understand my “illustrative” explanation).

So is there anyway to remove the “grandchildren” from the “parent” page, or does anyone else have any ideas how to solve this problem. I have very little php experience since i just started with it.