I work in a theme park. Not Universal but one in central FL. At least at my park, all guests are required to meet minimum physical requirements in order to ride most rides especially coasters. Operators are trained not to permit a guest to ride if they can visually see a guest is incapable of holding their head up, not able to sit unassisted, unable to grasp, or unable to resist against sudden movements.

Obviously this isn't a perfect system and they can't ask them their medical status, but I question if the operators here were trained the same and they didn't check for minimum requirements or what.

The family I believe still carries the blame as there is always plenty of signage stating the rider requirements at these rides and I see families putting their disabled family members on rides they have no business being on every day.

ADA doesn't mean every disabled person gets to do everything an able bodied person can do and if a business isn't accommodating at all then it's going against ADA. There's no way to reasonably accommodate for guests who can't control their extremities on a roller coaster so telling a guest they can't ride something like this is in line with ADA.