I reckon kids need to learn from early on that their bodies belong to themselves, and they have the right to refuse any contact or other things that they don't feel comfortable with. Even from their parents and others considered 'safe', because it's not always easy to distinguish between those that are 'ok' and those that aren't (and frankly, they're in greater danger from people they know than strangers).
It can make things difficult, but discussions of safety can work into this also, as obviously there are some rules that they need to follow for safety reasons. But if they don't want to play with x kid or kiss auntie or whatever, that's ok.
I like the idea of tricky people.
I think kids usually know when something's not right, they just also need to know that it's ok for them to say no and that they won't get into trouble.