The Thinkuknow website provides a huge amount of information, for both children and adults, across many aspects of online (and offline) child protection. It is highly recommended that churches make both children and parents aware of this site, and use it within their children's and youth work.
Parents Protect is a website created by Stop it Now and the Lucy Faithfull Foundation, and aims to prevent child sexual abuse through raising awareness and encouraging early recognition and response. It is highly recommended that churches make parents aware of this site, and encourage them to use it (particularly by creating a 'Family Safety Plan', which the website provides guidance for).
NSPCC Net Aware and NSPCC Share Aware are useful sites for parents to understand more of what their children do online and how to help them stay safe
The Ultimate Parent Guide for Protecting Your Child on the Internet