This website - www.internetmatters.org - is particularly informative.
If something has happened to your child which has made them feel worried/unsafe, or have been a victim or witnessed online abuse and is concerned, we advise parents to call 101 (Police Non-emergency number) or report it on the CEOP website - https://www.ceop.police.uk/safety-centre/
Update 1st May 2020:
Please be aware that the Government has released guidance for staying safe on-line:
Online safety guidance, advice and resources are also available from the County Council for parents, carers and young people.
Other helpful sources of advice are:
UK Safer Internet Centre https://www.saferinternet.org.uk/
Digital Parenting Magazine https//www.vodafone.co.uk/cs/groups/public/documents/webcontent/vfcon103583.pdf