Education is the best tool that a child can have. Encourage your children to think before they post online, and consider how they would feel if what they were sharing was made public. Making a plan with your child about what they should do if they do see something that upsets them is also a useful course of action e.g. stop, put the device down, tell an adult, use the reporting tools together etc.
You may find our Parental Controls, Pornography, and Video Chat and Webcams Hot Topics useful for further information.