Tower Controller providing OJT failed to intervene in a timely manner when the developmental was late in insuring separation between a departure and an over flight. Controller noted lack of sensory vigilance as contributory to the event.

Narrative

On job training in progress; CPC in training at 'dangerous' phase; doing well; but hasn't faced many unusual situations. Cessna 172 called from west side of Bravo airspace; wanting to transition through the departure corridor to east side of Bravo. I advised CPC/IT to bring Cessna closer to the airport; exiting Bravo sooner; to avoid departures. CPC/IT's transmission was not clearly understood by Cessna pilot; as evidenced by pilot read back. I realized this would soon conflict with our departures wake turbulence; as the Cessna would intercept the departures about 3 miles upwind; where departures are usually above 3;000 FT. I was letting the potential wake turbulence situation progress to a more obvious point; where there would still be time for the Cessna to turn and avoid wake turbulence. I made the mistake of not being vigilant with the RADAR; focusing instead out the window. Even though I was faced directly at a B737 and the Cessna; I failed to notice the B737 unusually slow climb; and Cessna's rapid progress towards our departure corridor. The Supervisor called our attention to the impending conflict. Next; when CPC/IT made the traffic call to the Cessna; it was too little; too late (approximately 1 mile lateral; 004 vertical; converging). The B737; already on Departure Control frequency; descended slightly to avoid a NMAC with the Cessna; most likely as a result of a TCAS alert. In other words; ATC (me) failed to separate those two aircraft. Further; our 'traffic call' to the Cessna should have been a 'safety alert' call. Recommendation; the instructor needs to be vigilant to all sensory inputs (RADAR; windows; sounds; etc.). The instructor needs to intervene after problem becomes obvious; but well prior to situation becoming dangerous. I was a full minute late. The instructor; though not required by FAA medical; should have vision improvement devices on hand (binoculars or prescription glasses). The binoculars were shared with CPC/IT. My (not required) glasses were not available.