Internet Matters is a UK organisation that works with online safety experts to guide parents through the many issues children can experience when using the internet. The site contains useful support and advice.
Think U Know is operated by CEOP (a branch of the Police National Crime Agency). The site is great for all ages and has sections for children, teenagers and parents. It is full of guides and advice for parents many aspects of online safety.
Childnet International is a non-profit organisation working with others to help make the internet a great and safe place for children. It is great source of advice for young people, parents, teachers and professionals.