D10 Controller providing OJT experienced a loss of separation when the developmental failed to issue decisive and timely corrective action involving two air carrier departure aircraft.

Narrative

My Developmental and I had two sessions on the Departure one position; and I am not certain in which session the event occurred. The two aircraft departed on the same initial route. The trail aircraft was the B757. The B757 climbed rapidly and increased speed relative to the MD80. As the situation developed the Developmental was exhibiting some indecisiveness; and I implored him to take action to insure separation if he felt a problem existed. He eventually climbed the trailing B757. I thought that we maintained vertical throughout; but while debriefing the Developmental later in the shift he indicated that the aircraft was a 1000 FT higher than I thought. I saw at one point 8300 on the MD80 while the B757 was out of 9400. The data tags were overlapped so it is possible I misread one or both. Regardless of the outcome of this event; OJTI's need (partially myself) to better balance the need for learning vs. a safe outcome. At times this is an extremely difficult call; but needs to be weighted towards maintaining separation standards. Perhaps a reminder to all OJTI's that this is a problem and although it is a difficult decision operate conservative over trying to stretch a trainee.