[Kileli]

I have no idea, but I have a guess

The bigger the rides, the taller you haver to be. Also, the more intense, the more saftey is involved. Coaster designers can not make many diffrent sizes for shoulder restraints, so I guess if some one is 48 inches, or whatever, they would fit into the restraints safley. For less intense rides, like BTMRR, kids under what ever height are obviously smaller around the waist. So, if you get your dad to sit next to you, and there is only one bar, it would go to his waist, not yours. At a point where there are negative G's, you have all that room to fly up and out. Not a fun thing to think about. Back to the shoulder restraints, maybe your feet don't touch the ground. It would be dangerous on a coaster (apart from floorless or inverted) to have your feet dangling.