15m is much too short. Register with thinkuknow.co.uk and download some of the videos such as Jigsaw and Consequences - these make effective tools with which you can bracket your safety message. The vids come packaged with safety advice and how to use them correctly. Kent is a very good source of material, especially their blog. Becta also have a mailing list called Safetynet which produces good material and CEOP participate there.
I normally deliver a staff session over about 45m, which includes the clips above (for primaries).