Our children's developing brains need strong moral input in order to grow well-ordered lives. Logical moral boundaries seem to have crashed in our culture, and today's children get much of their information/attitudes about morality and sex from popular media. Children may learn about sex on the playground or even from novels assigned at school. They may hear nothing about it from parents until "The Talk", which may come too late and be awkward.
On the other hand, many parents are taking a proactive approach, using purchased sex education curriculum. Close friends recommended a series, God's Design for Sex.
This series has four books written for ages 3 to 5, 5 to 8, 8 to 11 and 11-14. When their older child's school offered sex education last year, her parents pulled her out for those lessons and used the appropriate age level book to teach at home instead. All their children have enjoyed reading and rereading their age-level books in this series.
Also recommended as "very good" was a book for parents: How and When to Tell Your Kids About Sex: A Lifelong Approach to Shaping Your Child's Sexual Character
Hope this is helpful, as we start a new year, full of promise!