you can apply for a "one time" leave and re-entry certificate for around 290RMB...(forgot the name of that certificate) then it will temporarily solve the issue for the kids to go abround (for a family vacation or something) but "旅行证" is not valid anymore. That one time certificate can be used within 3 months after application has been filed but only for 离境一次再入境一次. (if you need more entries, just apply for another one and pay money again:-)