I know that with my little brother & sister, my mother sat them down and told them exactly what you have written up there: that no one can look or touch their private areas besides their parent and doctor. She reinforced it with them a few times, and when they went to the doctor for their yearly check up, she had the doctor reinforce it with them as well. This was at age 4, after an almost-scary incident; that is when she started talking to the kids about it and telling them that even if the person who tries to look/touch tells you scary things like ‘if you tell your mother, I’ll burn down your house’ etc, you should ALWAYS ALWAYS tell her anyway and she will not be upset.
I think that’s part of what you have to tell a little kid when you prepare them: that the molester will tell them things like ‘If you tell your parent, I will…’ and you just have to ignore it and tell your parent anyway. A lot of times kids won’t tell because they are afraid of the molester hurting them or someone else they know.