LAS Controller providing OJT failed to note a developing loss of separation involving two departures from parallel Runways 19L/R; the reporter indicating the Developmental was close to certification and his/her false sense of security regarding the Developmental's performance was a causal factor.

Narrative

I was providing OJTI (On the Job Training Instruction) to a Developmental that was a long way into training. The Developmental cleared Air Carrier X for takeoff Runway 19L; then proceeded to clear Air Carrier Y for take off 19R. The Developmental did not get visual separation. I looked at both aircraft. Air Carrier X was off departure end and Air Carrier Y was still coming around the corner to get lined up. On a normal day separation would have been more than adequate. However; there was a 20 knot head wind and Air Carrier Y was quicker off the departure end than Air Carrier X. A loss of separation occurred. After seeing both aircraft's position initially; I moved on to the next task; and failed to take a second look at the situation to ensure separation. When it was brought to my attention; Air Carrier Y was already switched to Departure Control and nothing could be done on my part to fix the situation. Recommendation; I think part of it has to do with a false sense of security. You're training a Developmental that is close to getting checked out on this particular position; traffic is light and there is an assist position open with a CPC (Certified Professional Controller) staffing the position. The particular Developmental had not been having any issues with departure separation. It is important to stay focused and to continue to scan a situation to ensure that what you anticipate to happen actually happens. Take into account aircraft performance when there is contributing factors such as wind; weather; or high temperatures. Be mindful of periods of underload and don't make assumptions based on past experience alone. Don't switch the second aircraft until separation is ensured. Get visual separation when in your mind it is going to be close. Give every aircraft and every situation the attention they deserve. Don't be so quick to move on to the next task.