You want to replace this:
WHERE $user_info[query_see_board]" . (empty($modSettings['countChildPosts']) ? "
AND b.childLevel <= 1" : ''), __FILE__, __LINE__);
With:
WHERE ($user_info[query_see_board]" . ($user_info['is_guest'] ? " OR b.ID_BOARD IN (12, 13, 27)" : ' OR b.ID_BOARD IN (12, 13, 27)') . ')' . (empty($modSettings['countChildPosts']) ? "
AND b.childLevel <= 1" : ''), __FILE__, __LINE__);
Just put your board number in both spots and you are all set, worked for me.