When and how should children be taught about bad touch and sexual harassment?

I am in no way asking if children NEED to be educated of the same. They HAVE to be. Expect vehement opposition from me if you feel otherwise.
Parents or expecting couples or even anyone else, when do you think would be the best time to explain to a child about abuse and what to do to prevent it and/or if and after such a situation arises? Most importantly, what method do you think should be taken up in order to get your point across without absolutely terrifying the child?

P. S. picture from pakistantoday. com. pk
When and how should children be taught about bad touch and sexual harassment?
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21 d
Thank you for all your responses. I'm only sorry I can't pick more than two MHOs. Then again, that wouldn't be MOST Honest Opinion then...
I wish you all the very best. Thank you for your time and expertise~
When and how should children be taught about bad touch and sexual harassment?
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