Following code will be useful if you want to apply parent page template to all the subpages without selecting page template from the options.

You have to add following code inside the function.php you can find this file inside your themes folder.

function change_page_template(){
	global $post;
	$current_page_template = get_post_meta($post->ID,'_wp_page_template',true);
	$parent_page_template = get_post_meta($post->post_parent,'_wp_page_template',true);
	$parents = ($post->post_parent==$post->ID) ? true : false;
	if($parents){update_post_meta($post->ID,'_wp_page_template',$parent_page_template,$current_page_template);}
}
add_action('save_post','switch_page_template');

So after adding these lines parent page template will be added automatically to all the subpages.