I really can’t improve on the lesson provided by Digizen.org.
This module was written for KS3 pupils but having been into primary schools which have had incidents of cyber bullying I would strongly encourage KS2 teachers to cover these issues with upper junior school pupils. It may mean that some pupils will cover this twice but that is surely preferable than pupils having no information if cyber bullied or being drawn into cyber bullying without realising the consequences for their victims. The lesson plan is for an hour but a discussion around the video can be covered in a shorter period of time. The video is excellent and I have used it with Junior age pupils very successfully.
The only other aspect I would stress is the importance of telling someone such as teacher or parent as soon as the bullying starts. Pupils will often not tell parents or teachers as they feel they will have their technology taken away from them. In effect the victim will be punished instead of the perpetrator. If parents or a responsible adult discusses this ahead of time and allays those fears children are much more likely to tell someone. An ongoing discussion around these issues with parents can help keep children even safer.
I have often done this as an assembly with the whole of Year 5 & 6