Small Aircraft
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 1.5
Situational Awareness
Conflict NMAC
Flight Crew Took Evasive Action
Human Factors; Procedure
I was conducting OJTI on the Local position.  The developmental had been on the ground position prior to the training session and was offered a break but declined it.  It was a nice sunny clear day with 3-4 in the VFR pattern consistently throughout the training session with steady departures and arrivals to the airport.  The developmental had been recommended for certification the prior week and told that if he makes a mistake; to fix it and continue working the traffic. At the time of the NMAC there were multiple aircraft in the pattern; departures to get out along with aircraft sequenced to land.  The developmental told Aircraft Y to continue upwind to the 60 freeway and that his crosswind was approved at the 60 freeway. The developmental forgot about Aircraft Y.   Aircraft X had been in the pattern and did a touch and go.   He told them to turn crosswind.  He turned Aircraft X right into the Aircraft Y and issued no traffic to either aircraft.  The Aircraft X advised tower about the Aircraft Y and that they came within 50ft of each other.  The developmental then realized what had happened.  It was discussed and briefed at a later time.When you see your developmental struggling to keep afloat offer them a break and breather.  Not let your developmental take things so far out that you as the instructor can't keep up with what they're control judgment is and be more aware and on top of what your trainee is doing.
EMT Local Control Instructor reports while training; he let his Developmental get two aircraft get too close together; causing an NMAC report by one of the pilots.