It is essential that the work can not injure adults or children, even it they abuse it.
People will tend to spin things as fast as they can go; the system must resist that by some means.
If a work has large moving parts they must not be able to be moved to where they would hit a person.
This is why our first works consist of cans and balls on steel rods: so the motion is very limited. Each can and ball has objects inside that slow down the rotation of the ball or can (which also allows people to adjust them more accurately.)