I swear like a your-favorite-stereotype. When my kid was maybe 4, I told her "I don't care if you talk like I do, but people will give you trouble for it". Probably with more detail than that; it's been years now.

She fully understood the point, right then. She also had no problem with other advice about how people would react to whatever.

She's 17 now. I actually don't think I've ever heard her utter a "swear word".

Kids, in general, have no problem with the idea of social context.