[ Ref. Matthew Caulfield in 161116 MMDigest ]

The term "spotter" is commonly used in sports such as gymnastics
to refer to a person who helps ensure the safety of participants.
For example, if you're learning to do back-flips, the spotter stands
beside you and may support your back, or be prepared to do so, to
ensure that you don't fall and hurt yourself.

It's a hands-on role -- the spotter observes closely and physically
supports the athlete, or is prepared to do so, if needed.  The role
that you're describing for the carousel is similar: someone who is
looking out for the safety of riders.

Meta Brown
Chicago