BTE, I agree that you should probably seek help in finding out why you are having these fantasies. Although I've heard of rape fantasies, your fantasy of seeing a man sexually abuse a child is a bit different. The only reason I haven't concluded you are personally a child predator is that you fantasize about a
man molesting a child and you're a female. Is there a possibility that you were abused as a child or that you saw it happen to someone else? However, if you find the idea of
you abusing a child arousing, please get serious help immediately. There is definitely a reason behind these thoughts, and in order to have a healthy sex life, you need to find out what is making you tick this way.
From what I've read, there are a couple of explanations for the rape fantasies. For one thing, if you are normally a dominant or controlling person, that could bring out the desire for someone to dominate you... hence the rape fantasy. Second, sexual guilt could play a role since your fantasy is your own creation... therefore you are seeking punishment for your sexual desires. I don't know that any of these apply to you, and the voyeuristic molestation fantasy is a new angle I've not heard about before.
My advice? Go see a shrink and look for a book that might shed some light. Your fantasies are not exactly within the bounds of normality.