List child pages, exclude the current page

Looks like a syntax error to me. Try:

wp_list_pages("title_li=&child_of=2143&exclude=$current_post_id")

OR

wp_list_pages("title_li=&child_of=2143&exclude=".$current_post_id)

Also I will suggest to pass parameters as array instead of a string, for better debugging.

wp_list_pages( array(
    'child_of'    => 2143,
    'exclude'  => array( $current_post_id ),
); 

Make sure $current_post_id is giving proper value.