A B767 FO RPT REGARDING A TKOF WITH SOME CABIN ATTENDANTS NOT SEATED OR STRAPPED IN AT PHL; PA.

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PUSHBACK AND TAXI WERE NORMAL. AS WE WERE CLRED INTO POS AND HOLD; THE 'F' FLT ATTENDANT FOLLOWED BY THE 'A' FLT ATTENDANT CAME INTO THE COCKPIT TO TELL US ABOUT 2 DISRUPTIVE PAX. THEY WANTED THE FLC TO TALK TO THEM. AT THIS POINT; I HAD ALREADY NOTIFIED THE FLT ATTENDANTS TO TAKE THEIR SEATS FOR DEP AND WE WERE LINED UP FOR TKOF. THE CAPT ASKED THE 'F' FLT ATTENDANT IF IT WOULD BE OK FOR US TO TAKE OFF AND THEN TALK WITH THE PAX. HE SAID OK AND THEY BOTH LEFT THE COCKPIT. JUST AT THAT MOMENT WE RECEIVED OUR TKOF CLRNC AND WE DEPARTED. PASSING 10000 FT; THE CAPT WENT BACK TO THE CABIN AND THEN DISCOVERED THAT THE 'F' FLT ATTENDANT HAD NOT BEEN PROPERLY RESTRAINED FOR THE TKOF AS WERE SEVERAL OTHER FLT ATTENDANTS. THEY THOUGHT THAT SINCE THE 'A' AND THE 'F' FLT ATTENDANTS WERE IN THE COCKPIT; WE WERE GOING BACK TO THE GATE AND WERE NOT IN THEIR JUMP SEATS. THIS WAS A VERY EXPERIENCED CREW; BUT THERE WAS STILL MISCOM. THE FLT ATTENDANTS DID NOT REALIZE HOW CLOSE WE WERE TO TAKING OFF; EVEN THOUGH I MADE THE ANNOUNCEMENT; AND DID NOT GIVE US AN INDICATION THAT THEY THOUGHT THE SIT WAS SERIOUS ENOUGH TO NOT TAKE OFF. WE DIDN'T REALIZE THAT THE 'F' FLT ATTENDANT WAS SEATED IN THE BACK OF THE ACFT AND DIDN'T HAVE TIME TO GET TO HIS SEAT. IN THE FUTURE; IF A FLT ATTENDANTS FEELS THEY HAVE ANY PROB IN THE BACK AFTER OUR 'BE SEATED' ANNOUNCEMENT; THEY SHOULD BE DIRECTIVE ABOUT NOT TAKING OFF SO THAT WE CAN GIVE THEM ENOUGH TIME TO EXPLAIN; AND US TO EVALUATE THE SIT.