Statistics vary pretty widely. 15 out of 100 is a pretty common. 1 in 4 girls, 1 in 6 boys experience sexual molestation by the time they are 18. The problem is in estimating precisely how under reported it is. At least 3 million by age 13 is a relatively safe bet.
The issue, as you'll see if you Google it for articles, is that children are generally taught to be obedient and are not taught about sex. So they actually have no idea that what the adult is asking them to do is wrong or unusual. Sometimes it isn't till years after the abuse has stopped that they'll realize what really happened.
I could really contribute some pretty awful stories to this thread... human sexuality fascinates me and throughout my life most of my friends have been women, so this is the kind of thing I talk about a lot.
Here is one. Girl was living with parents and her maternal Grandmother. Maternal Grandmother has dementia and is also insane. Kidnaps the little girl, takes her to a motel, and sodomizes her with a coke bottle for several hours. Glass coke bottle. She was 6. Her Grandmother wasn't left alone with the kids after that and her Mother started drugging her food with aspirin (I know this makes no sense) and would then sneak into her room at night and whisper that "Nothing had happened. You're a happy little girl, nothing happened." Stuff like that. This went on till she was 8. Then her two oldest brothers started molesting her, Grandmother died, and the Mom just... gave up and let it happen.
Turns out, the Mom had been molested by the Grandmother growing up and had been forced to "make herself available" to her brothers, so she just... accepted it as a way of life. That was "normal" for her and I guess dealing with it would have forced her to deal with her own childhood issues. No one is sure if the Grandmother was also molested in a like fashion or if it started with her.