Do not show all post from all categories, just show posts from category in current loop

You are wrongfully handling the WP_Query parameters. You can either use a query string type input or an array based one. Personally I much prefer the array type, so my example uses it:

$category_id = (int) $child_category->term_id;
$des_post_args = array(
  'post_type' => 'tour',
  'cat'       => $category_id
);
$des_post = new WP_Query( $des_post_args );