It seems that apart from Overstress nobody has kids here. Mine are 2,4 and 6. We've been taking the plane since their birth and sometimes we went through difficult moments when they were anxious or sick. We always tried our best to calm them down or make sure they don't cause too much disturbance in the cabin. Here we have a kid who's affraid and who's accompanied by his grandparents and not his own parents who cannot talk yet and is probably still wearing diapers. If the grandparents have problems to make sure the frightened kid puts on his belt than surely one, and not the three of them together, ca must be able to do it. Just do it, take the kid, strap him in, end of story. No brutalities or discussions needed. Trying to convince a two year old is like carrying water to the sea.
Much to do about nothing.