TRACON controller providing OJT to a supervisor experienced an operational error when the developmental tried to clear the arrival path for the emergency; the reporter suggesting that only supervisors or senior controllers train other supervisors.

Narrative

I was training a Supervisor Developmental. This was the first or second time I have trained someone on a RADAR position and I have asked repeatedly not to have to; but I was asked to anyway and advised that the Developmental was close to being done training; so I agreed. Aircraft Y was inbound to Runway XXL and declared an emergency; having lost an engine. The Developmental was trying to get the smaller aircraft out of the way and I was assisting somewhat. Aircraft Y was cleared on a heading in to be first and followed by an air carrier on a northwest heading; changed to a 360 heading. Because of the engine loss; Aircraft Y did not descend very quickly and when realized that there was a problem the Supervisor in Charge said to turn the air carrier. I overrode the Developmental and turned Air Carrier X to a 040 heading. Then; I issued traffic to Air Carrier X and he said he had it; so I told him to maintain visual separation with the Citation. He said something like OK. Recommendation; I personally wouldn't train on RADAR. I would suggest that only very senior controllers or supervisors train supervisors. Perhaps some sort of altitude restrictions with emergencies or having aircraft in holding or something like that would also help.