Most adults don't have a problem with swearing among each other, but for some reason it's still considered scandalous to swear in front of a child, and possibly worse when a child themselves swears. This idea is baseless and deeply harmful.
Words are arbitrary sequences of syllables agreed to communicate something. The word "fuck" for example expresses anger or other negative emotions, which are, of course, a ubiquitous part of life and not "bad" at all. Saying "fuck" does not make a person angry or violent any more than searching for a job makes someone unemployed - in fact, such language provides valuable catharsis you can't get from any other words. Saying "I'm very angry" sounds particularly not angry.
What happens when you discourage yourself and others from using these words when you would like to? The same thing that happens any time you bottle up a negative emotion: it gets stronger and eventually will be expressed in some form or other. Training oneself or others not to express emotions is wrong and harmful. Training only some people to feel disallowed from expressing their emotions is even worse, because asymmetric rules are always worse than no rules at all.
There is such a thing as a "bad" word. Here are a few examples:
Words that represent a concept that isn't real, like "redeem" which because it's grammatically transitive creates the incoherent idea that you can be redeemed by someone else's actions
Words that represent a concept already represented by another word giving the impression that there's a difference where there isn't ("person" and "mind")
Words that represent multiple different concepts and muddy our thinking, like "good" which refers to both morality and happiness - wonder why so many people are utilitarians?
Swear words don't fall into any of those categories. They are in fact very good words because they do in 1-2 syllables what is difficult or impossible without them; and there is no justification for a double standard against children using them.
This doesn't mean we should use profanity all the time; its usefulness is to convey things stronger than can be done with normal words, so if you use it too often, it starts to lose its impact. It's wiser to use profanity sparingly so that we have access to extreme language when we need it. (I may be a hypocrite on this.)