Nah, piece of cake...
Not!
My son was 7 years old. He was talking about things that happened at his school - he was living with his mom in the Cape at the time - and something he said made my ears prick up. And I questioned him carefully, only to finally realise that a child (almost double his age) at the aftercare (son of the owner) was molesting the other children.
He'd told his mother exactly what he told me, but she just pooh-poohed it and kept sending him to that place. He kept saying, "But mommy doesn't BELIEVE me - she won't LISTEN to me!" Literally, his mother was delivering him to a molester every day, and no matter how much he protested, she just kept right on doing it.
So I hauled out the textbooks, and told him everything, from top to bottom.
I showed pictures. I made drawings. I found photographs - ANYTHING he needed to help him understand and grasp the mechanics of the matter. And I promised him that, no matter WHAT happened, he could always count on me - and no matter what he wanted to know, I would always tell him the truth, and if I didn't know, I would find out for him. I've kept that promise.
Oh, and I told him to tell that child that, if he EVER touched my son again, I would fly down to the Cape and personally rip his head off with my bare hands. That little line had an electrifying effect, apparently...
Of course, a week after he'd gone back to the ex, I got a phonecall from her telling me I was disgusting, perverted, and sick for telling 'her' son about the birds and bees, and she even threatened to lay charges against me - to which I replied "If you had done the job of parenting OUR son properly, the way you seem to believe only you can do it, and LISTENED to him when he cried out for your help, our son would not have been repeatedly exposed to a child molester at his school. Do me a favour and lay those charges, and let us see exactly where you get with that. If ANYBODY should be prosecuted, it's you for not doing your job properly! So go ahead.".
That was the last I heard of the matter until recently - he's now 19 and living with me, despite his mother's efforts to stop him doing so - when he opened up and told me exactly WHAT had happened and how my lecture to him had given him the strength to say "NO! YOU WILL NOT TOUCH ME!!!!!". He also told me how his mother had THEN listened to him, far too late to help, but only after I had taken her on.
Teach them about the birds and bees earlier rather than later, I say, but it always depends on the child him/her self.
Just make sure you give them the FACTS, ALL the facts, and nothing BUT the facts!
Don't try to bullsh1t them - kids can smell that from MILES away.