"Oh yeah," I replied, expecting it to be something like bumhead or weeface, the recent phrases of choice to describe me when they don't get their way.
"It's f*cking," he replied with a smile.
"Yes, it's f*cking and you always say it," added his brother just to make sure.
Now, if this sounds exactly like the incident described in this link to the ways in which imitation works in children's language development, then that's probably because it happens to a lot of us. But as the article goes on to say, imitation is only part of the story. Kids will pick up swear words but it's the social interaction with parents and peer group that determines how swearing is used and whether or not children understand the impact of what they're saying.
As the article puts it, quoting psychologist Paul Bloom:
Another part of growing up is knowing how to speak with adults and in formal situations. "So we'd like our children to grow up knowing when it's appropriate to use these words," Bloom says.
As most parents come to recognize, teaching good judgement is not a one-time event; it's a process.
ENA1 - Child Language Acquisition