Hmm!
I'm a parent.
I'm also a teacher.
I see what happens when parents DON'T talk to their children about sex at any age.This is possibly why we have the highest rate of teenage pregnancy in Europe although religion and culture may also play a large part in the differences between countries.
DON'T leave it to school, teach your children about this yourself.
Children are maturing much earlier than they were in my day.I began my cycle when I was 12 and I was unusual.Now it is much more common for girls to begin puberty much earlier than my generation did.They need to be forwarned of all and every consequence.
I had another baby when my eldest was 5.I began to talk to her about where babies came from when she asked why I was getting fat(ter).I had to keep it simple so I got Clare Raynors book for young children with lots of pictures.Children will ask simple questions at this age and expect simple answers.They don't need long words so use the ones they understand.
I do not as a parent and as a teacher subscribe to the 'keep them ignorant and teenage pregnancies will go down in number' as this never worked BEFORE sex education was provided in schools.
I know no parent likes to contemplate the thought that their child will one day become sexually active.However the day will come when they are.Better that you have protected,warned,advised and helped them rather than leaving it to 'experts'.There is only one set of experts when it comes to your children and that is you as parent/s.
Incog.