"Offense is a choice made by the listener to be offended." Actually, no. It's an instinctual reaction over which people have very little short-term control.

"Swearing is just a bunch of sounds put together, and by themselves they are meaningless, not some sort of magical phrase." I'm sorry but that's nonsense. Spoken language as a whole is the assignment of meaning to sounds. The sounds may well be meaningless as isolated syllables, but they're not being spoken as isolated syllables. They're being spoken as language, and spoken language has meaning. Its whole point is to have meaning. And, hey, "fuck" is an isolated syllable and it's already offensive without being combined with anything else.

@DavidRicherby: Is the sound "fuck" offensive due to the meaning that is attached to it in the English language, or are you saying that that particular sound is offensive in any language, including the ones that don't assign a meaning to it?

@BartvanIngenSchenau Within the context of the English language, the sound and meaning are inseparable. If the toddler had been taught to swear in some language that most of those present didn't speak, it would cause less offence, but that's not what happened.

You've sufficiently ranted about the way in which parents normally handle inappropriate language. While it may be valid, you only have a little, tiny, one-sentence paragraph that even addresses the actual question. If you could expand on that to actually provide examples of what to do, instead of what not to do, this would be a good answer.

@DavidRicherby: OK, it's not a conscious decision, but can learn to be offended and learn not to be. It is a choice that few consider. The 'magical phrase' thing just means that the sounds do not invoke some power of the universe, they are just sounds that you interpret. People treat swearing as though it is an objective cause of offense, when it is entirely subjective.

@Clay07g: The substance of the question seems (at present) to be about the 9 year old, not the toddler. You can't parent a stranger's toddler. I would explain the use of language to a 9 year old.

@AzorAhai: That is almost the opposite of the meaning of what I've written, so I'm sorry I haven't been clear. My point is that words have meaning in a context, not by themselves. So swearing is not universally bad, it is a choice when you have a particular audience. I haven't, for example, sworn in the answer, though I could easily have done so for effect. The sounds themselves are not the issue, which is my point.

@DavidRicherby What I have to say is completely irrelevant to OP's question or this answer, but your comments are philosophically profound to me. My initial reaction was dismissal of the idea that language is an assignment based construction of meaning, specifically due to the historical existence of phenomenon such as lexical ambiguity in homonyms/homophones (see buffalo*), euphemistic treadmills, and reappropriation of emotionally laden vocabulary that gives rise not only to meanings that evolve over time but also completely invert the "original" emotional connotation.

@DavidRicherby In a computational complexity view, I'd argue natural language is actually construction via negotiation which is an iterated game over time across generations and between individuals that has some goal maybe such as Zipf's hypothesis of the principal of least effort in expending resources to establish meaning in concert with each agent's internal exploratory goals and reward-seeking actions through repeated interactions, or maybe evolutionary fitness explanations, whichever.

@DavidRicherby The point is that most units of symbol combinations never exist or are truly meaningless, eg. Library of Babel, giving weight to the idea that every combination of symbols we do speak due to their relative scarcity amongst potential choices are in fact very concretely meaningful but whose meaning ultimately does not arise from a physical link to a state of mind or configuration of environmental objects but a metaphysical computable process of agents acting on a common communal tape to extract useful information and act upon it according to their goals.

@Clay07g: I've added a section at the end, though I'm not particularly satisfied with it. If it doesn't address the question, could you expand on what you believe the question is?

Answer is irrelevant. Poster stated that they were told to F-Off by a toddler. This isn't a choice about offence, that is a sign and spirit of extreme disrespect, which is the nature of the dilemma.

@NOP the question isn't about the parent being disrespected, but about their own child. (If that's even what it is; we're talking about a 3-year old using words they know will generate a reaction here)

@EpicKip If it's always the choice of the listener to take offence, why bother being offended by people yelling at you? I mean, seriously, you chose to get offended by people yelling that you're a Rice Crispie? I have to say, that's a really dumb choice. And if being offended is such a choice, why do you never hear anyone say something like "That's really offensive! Actually, I've changed my mind. I'm fine with that."? Do you believe that other emotional responses are also choices? You just lost your job? Don't "choose" to be sad -- you could "choose" to be happy instead!

@DavidRicherby Note the word kept. I don't get offended easily but if someone keeps screaming (no matter what) I'm bound to get pissed.

@DavidRicherby My point is that swearing is never by default offensive. I swear all the time when I'm with certain people, they do too. No one means it as an insult or anything and no one takes it as such. So yeah it's a choice....

@EpicKip Sure, it's context dependent. But that's a completely different thing from being a choice. You're not offended by your friends' swearing because you recognize from the context that it has no offensive intent.

"If I stand in an empty room and shout obscenities and no one can hear, does it matter? No." As long as the rooms near you can't hear you either. A couple of years ago I sat in a train while 2 people in the coupé next to me (which was empty except for those 2 individuals) where swearing up a storm at nearby traffic. The traffic couldn't hear them, but the rest of the train could.