child page navigation disapears

hi,

my page shows a main navigation and a subnavigationcincluding all “pages”.

for the subnavigation (childpages) i use this
<?php wp_list_pages(“exclude=25&child_of=$post->ID&depth=2&title_li=&sort_column=menu_order”); ?>

but when i click on a childpage the subnavigation disappear. which is just logical, because this “child_of=$post” try to display the child pages of the shown child page, which are not existing. is there way to say show “childpages of the main page”?