How to pull data from child installation to parent theme

You want to enable WPMU and make custom functions to get/post data from one blog to another (using switch_to_blog). WPMU installs different tables for each blog you create on one WP installation.

If you want to have the same design, child theme can be enabled on one WP installation. You cannot switch between child and parent theme just to get different data. You should clear things up.

Example: (not complete – i use more to do my work)

function copy_data_to_other_site($post_id)
{
    $current_user = get_current_user_id(); // get user id

    $target_blog_id = get_user_meta($current_user, 'primary_blog', true); // get blog id by user

        $post = get_post($post_id, ARRAY_A); // get the original post

        $the_slug = $post['name'];

        switch_to_blog($target_blog_id); // switch to target blog

        $args = array(
            'name' => $the_slug,
            'post_type' => 'post',
            'post_status' => 'publish',
            'numberposts' => 1
        );

        $my_posts = get_posts($args);

        $target_car = wp_insert_post($my_posts); // insert the post

        restore_current_blog(); // return to original blog
}