It's very difficult to shield a kid from cyber-bullying. I don't have children myself, and I always say that when I will, they won't have a smart phone or a tablet until they are old enough to buy one. But I think that is very naive and desn't solve anything. Nowadays anyone can get access to the internet from anywhere.
I was bullied as a kid in school, and home is the only place I felt safe. Now you have a computer or phone at home, and the bullies have access to you anywhere. It's very scary, and frankly I'm not sure it's possible to avoid. I guess the best way is to educate children about posting stuff online...