"The most important 50 minutes in a child's school career"
OK, maybe that's a bit over the top - you can only do so much in 50 minutes.
However, our experience in many schools is that your 50 minute talk may be the only time in a child's entire school career, from 5 to 18, that a non-theist world view is presented by someone who shares it.
So, it is vitally important that this short time is used to best effect. You need to give pupils food for thought to help them decide for themselves what they will believe as adults.