/**
* Return whether the current page is a child of $id
*
* Note: this function must be run after the `wp` hook.
* Otherwise, the WP_Post object is not set up, and
* is_page() will return false.
*
* @param int $id The post ID of the parent page
* @return bool Whether the current page is a child page of $id
*/
function is_child_of( $id ) {
return ( is_page() && $id === get_post()->post_parent );
}