function wc_update_300_grouped_products() {
	global $wpdb;
	$parents = $wpdb->get_col( "SELECT DISTINCT( post_parent ) FROM {$wpdb->posts} WHERE post_parent > 0 AND post_type = 'product';" );
	foreach ( $parents as $parent_id ) {
		$parent = wc_get_product( $parent_id );
		if ( $parent && $parent->is_type( 'grouped' ) ) {
			$children_ids = get_posts(
				array(
					'post_parent'    => $parent_id,
					'posts_per_page' => -1,
					'post_type'      => 'product',
					'fields'         => 'ids',
				)
			);
			update_post_meta( $parent_id, '_children', $children_ids );

			// Update children to remove the parent.
			$wpdb->update(
				$wpdb->posts,
				array(
					'post_parent' => 0,
				),
				array(
					'post_parent' => $parent_id,
				)
			);
		}
	}
}