My daughter did this when she was about three. Thankfully we had already taken off, but it was a brutal flight from JFK to DCA. She just decided she didn't want to wear the seatbelt, and there was nothing that could be said or done. Screamed every second she had to have the belt on after takeoff.
Thankfully that flight was quick. She received corrective action in the restroom between flights and was fine on the next leg.
Flight attendants and other passengers around us were great. I think people generally realize that in most of those situations the parents want the kid to stop more than anyone.