I'm not fond of calling centrifugal force an "imaginary" force. I prefer to explain it in terms of reference frames:

In an inertial reference frame, Netwon's laws are sufficient to model (non-relativistic) motion.

In a non-inertial reference frame, additional forces (like centrifugal, Coriolis, etc.) must be added to the model.

What makes centrifugal different than centripetal is that centripetal force is due to interaction with another object (gravity for orbiting objects, tension for spinning a ball on a string, friction for an object on a spinning merry-go-round, etc), while centrifugal force must be added to the model "out of nowhere", just because you are ina rotating reference frame.

To explain this to an 8th grade class, I would give an example of two physicists: 1 standing on the ground, one spinning on a merry-go-round. They both perform measurements to detect the forces acting on the spinning physicist. The physicist on the ground only detects centripetal force, and notes that this causes the other physicist to travel in a circle. The rotating physicist detects centripetal force and an equal but opposite centrifugal force, resulting in him remaining at rest in his frame.

I'm not fond of calling centrifugal force an "imaginary" force. I prefer to explain it in terms of reference frames:

In an inertial reference frame, Netwon's laws are sufficient to model (non-relativistic) motion.

In a non-inertial reference frame, additional forces (like centrifugal, Coriolis, etc.) must be added to the model.

What makes centrifugal different than centripetal is that centripetal force is due to interaction with another object (gravity for orbiting objects, tension for spinning a ball on a string, friction for an object on a spinning merry-go-round, etc), while centrifugal force must be added to the model "out of nowhere", just because you are in a rotating reference frame.

To explain this to an 8th grade class, I would give an example of two physicists: 1 standing on the ground, one spinning on a merry-go-round. They both perform measurements to detect the forces acting on the spinning physicist. The physicist on the ground only detects centripetal force, and notes that this causes the other physicist to travel in a circle. The rotating physicist detects centripetal force and an equal but opposite centrifugal force, resulting in him remaining at rest in his frame.