This is how I see it
Guy drives into 30 zone at 40mph and I don't flash him - drives right past the speed camera and continues to speed through this area all week until he gets his surprise through the post. The camera did nothing to prevent the driver from driving through the 30 mph zone at an "unsafe" speed and therefore all the children in the area were at risk
However if I were to flash
Guy drives into 30 zone and notices a friendly motorist warning him of his "unsafe" speed and as such lowers it to a "safe" speed. The children are now safe, and will more than likely be safe in future as the driver now knows that a camera is likely to be lurking in the bushes...
The flashing scenario is safer, just doesn't earn the £££