Like all amusement parks, the Disney parks have height restrictions on their rides to protect the safety of smaller guests, typically children. Riders who are "too small" run the risk of slipping free of restraints and being thrown from a ride.
The Disney parks have several rides for which there is no minimum height requirement, though some of these attractions have intense moments for kids, such as the Haunted Mansion and "It's Tough to be a Bug" movie. Children at least 54 inches tall can ride any ride without an accompanying adult.
Disney allows adults to "swap" spots on a ride while waiting with a child. If people are in line for a ride for which an accompanying child does not meet the requirement, one adult can wait in a special area with the child while the other adults ride. After their ride ends, the waiting adult can take their spot and enjoy the ride.
Disney has a redundant system to ensure heights are measured properly. Children will be measured by a cast member at the entrance to the queue and again by a cast member at the loading area.
