You can look into the wp_insert_post() function. It takes a $post array, and you can set the parent ID. here's the link: http://codex.wordpress.org/Function_Reference/wp_insert_post
here's an example:
// Create post object
$my_post = array(
'post_title' => 'My post',
'post_content' => 'This is my post.',
'post_status' => 'publish',
'post_type' => 'page',
'post_author' => 1,
'post_parent' => [ <post ID> ], // Sets the parent of the new post, if any. Default 0.
'post_category' => array(8,39)
);
// Insert the post into the database
wp_insert_post( $my_post );
I hope it helps! --updated with example