I admit that I know that a 4 wheeler is intended to be ridden solo as were the 3 wheelers (really dangerous machines) BUT I have often been given a ride on the back of a 4 wheeler and have hauled my wife on the back of a 3 wheeler. A giant difference in safety is due to the operator and operating circumstances.
Doubling on a 4 wheeler we never do anything remotely dangerous as regards speed and turns for the terrain and with wives on the backs of 3 wheelers we were out for a very sedate cruise on a deserted beach.
Note: 4 wheelers and such when operated in "Thrill Mode" should never ever have twp riders and that goes double for those times when there is a youthful operator. It really gets scary when the equipment being operated has fast moving blades.
Taking kids for a ride on dangerous equipment is not smart. How many children have bounced out of the FEL bucket and been injured? Too many. When mowers, tractors, etc are intended to carry passengers the MFG will provide multiple seating.
The neighbor giving the kid a ride on the mower is ignorant at best and worst case is stupid. My neighbor used to shoot 100 yd rifle targets with his 308 whiile his wife was mowing the lawn. He'd squeeze off shots when she cleared his scope. Later he did the world a favor and ate his pistol.(revealing his nature to the slow to comprehend set.)
Pat