This site has a comprehensive list of books and other resources for a variety or ages on sex and relationships.
From the NHS in the UK, this site has a wealth of information, including how to talk to about sex with children, teens and why it's important.
The FPA, also in the UK, has a similar site for parents and carers that is even further broken down by age, explaining where to start talking about healthy relationships with children as young as 3, all the way up to teenagers. It also includes information about speaking from a faith based and cultural perspective.
Cory Silverberg, Sex Expert and Author of 'What Makes A Baby and 'Sex is a Funny Word''gives tips and advice on how to talk to young ones about sex.
Not sure how to start a conversation about sexual education with young people? AMAZE provides videos are resources for how to do so appropriately and effectively. Check out Having the Talk series: