This happened all the time back when I worked there in 2005. The weird thing is that if this a real video, several Cast Members are to blame for not catching it. When you first get into the elevator and sit down, the CM loading the vehicle has a panel on the right side that show each seat with a light that shows if the seatbelt is fastened. Therefore, he can see if if someone is sitting there without a seatbelt on.
However, this panel only shows if there is a seatbelt locked in. What it doesn't stop is someone sitting on their belt. What the kid probably did was fasten the belt before he sat down, therefore his seat was lit up. What the CM failed to do was to have everyone "put your hands up!" and visually check if they can see a seatbelt over everyone's hips.
Then, the ENTIRE ride is being watched by someone in a room with cameras, and their sole purpose of the time they are in there is to have their eyes glued to the screens, looking for anything out of the ordinary. What bothers me is that almost the entire drop sequence was taken with the kid very obviously flying out of his seat and no ride stop of any kind was initiated. The kid was obviously very stupid, but for 2 Cast Members to miss a very important safety procedure bothers me too.