i cant get a clear answer if a final fantasy 13 demo will be available on the us copy of advent children, if anyone has bought it or know if the demo is there please let me know thanks

The answer is no. Square Enix has gone on the record as saying the FFXIII demo was exclusive to the Japanese version of Advent Children.

If we see a demo in North America, it will very likely be a downloadable demo, but there is no concrete answer to that yet.