When I was at Walt Disney World a few years ago I did notice an older kid who was around 5 or 6 wearing a pampers diaper under his shorts.
I could tell it was a diaper and a pampers diaper because of the prints of the diaper when he bent over and his shirt came up.
I don't know why he was wearing the diaper though and I didn't really care why, but some kids from 5 to 6 do pee their pants when they can't make it to restroom in time so that could be a reason he was wearing the diaper to Walt Disney World.
But then again he could have bladder problems and needs diapers.