One of the scourges of the world that I abhor is bullying, especially when it affects children. Very often a child can suffer without the parents being aware of what is actually happening to their child. It is the same online as it is in the playground.
GetSafeOnline (there is a permanent link at the foot of the page) has a section dedicated to giving advice on keeping children of different age groups safe from potential dangers such as cyber-bullying and cyber-stalking. If you are a parent it may be beneficial to you to read the advice on offer.
Other UK links: