WDB ACR ACFT TAKES OFF WITH PAX NOT SEATED OR BUCKLED WITH SEAT BELT.

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TAXIED OUT FOR TKOF. TWR CALLED TO CLR US INTO POS AND CLRED US FOR TKOF. AT ABOUT SAME TIME; A FLT ATTENDANT KNOCKED ON THE COCKPIT DOOR AND TOLD US A PAX HAD JUST GOTTEN UP AND GONE INTO THE BATHROOM. THE SO AND I WERE RUNNING THE CHKLIST FOR TKOF. THE CAPT STATED THAT WE WERE GOING TO CONTINUE. THERE WAS LITTLE TIME FOR DISCUSSION AND WE CONTINUED THE CHKLIST AND TOOK OFF. AFTER GETTING AIRBORNE; WE FOUND OUT THE PAX IN THE BATHROOM WAS REALLY A BOY AND HIS GRANDMOTHER. LOOKING BACK; I'M NOT SURE WHAT I COULD HAVE DONE EXCEPT TO XMIT TO THE TWR THAT WE COULD NOT TKOF BECAUSE WE HAD A PAX UP; THUS TAKING AWAY THE OPTION TO TKOF; BUT THINGS WERE HAPPENING SO FAST I DIDN'T THINK TO DO THAT. THE CAPT WAS WELL AWARE OF THE REGS. I GUESS I WAS IN DISBELIEF THAT HE WAS REALLY GOING TO TKOF WITH SOMEONE UP. ALSO; THIS PARTICULAR CAPT IS VERY AUTOCRATIC IN HIS DECISION MAKING. DISCUSSION OF THE SIT WOULD NOT HAVE HELPED. MY ONLY OPTIONS; IF I HAD BEEN QUICK THINKING ENOUGH; WAS TO ADVISE THE TWR OF OUR SIT AND TAKE AWAY THE OPTION OF CONTINUING.