If the child is old enough to spoken to, I find that being told rather firmly that the seatbelt must be worn often works because it's a) a stranger telling them and b) someone in uniform. Quite often mum & dad are doing their best but as we know little kids are little kids and sometimes just don't want to listen.
For example, little Johnny running up and down the aisle numerous times, asked parents to keep him in his seat, they didn't. Little Johnny just about knocked me over while I was carrying a coffee pot. looke Johnny in the eye (he was about 6), and said quite sharply, but not too meanly "Go sit down and put your seatbelt on, NOW". He did. For the rest of the flight. So sometimes they will listen to the crew because the crew seem more 'in charge' in the plane.
Of course if the kid doesn't want to listen and kicks up so much fuss (as this one obviously did) that it cannot be calmed or made to sit down then offloading may be the only option at that time.
I've also found with older kids that telling them how it works helps... e.g "We can't take off until I tell the Captain that everyone has their seatbelt on, and you haven't got yours on, you don't want everyone to be late do you?" or "The Captain can't take off until all the seatbelts are on and you don't want me to get in trouble do you?" often work wonders on older kids
Totally agree with sentiments above re: car seats & kids, some parents play 'dumb' but the car seat argument totally demolishes that ploy
