Theme Horse

Hi ptp,

Create a functions.php file and first unhook the functions then only customize the code in your child theme.
example for page 404 to unhook :-

unhook functions

// Unhook default Thematic functions
function unhook_thematic_functions() {
    // Don't forget the position number if the original function has one

remove_action( 'interface_404_content', 'interface_display_404_page_content', 10 );

}
add_action('init','unhook_thematic_functions'); // removes the header content by using hook interface_header

add_action( 'interface_404_content', 'interface_child_display_404_page_content', 10 );
 
function interface_child_display_404_page_content() {      ?>   
<div id="content">
		<header class="entry-header">
			<h1 class="entry-title"><?php _e( 'Error 404-Page NOT Found', 'interface' ); ?></h1>
		</header>
		<div class="entry-content clearfix" >
			<p>Thank you</p> 
		</div><!-- .entry-content -->
	</div><!-- #content -->
  <?php 
}

Hope this code may help you to customize the code and also view this codex
http://codex.wordpress.org/Child_Themes

Thank you!