This point was driven home to me when my own son was about four. Stuck in our tiny coach seats seemingly forever, on the tarmac somewhere waiting for something or other to be fixed, I was reading him the same three books over and over, trying to keep him amused. I was about to sail into "Where the Wild Things Are" for like the third time, when Zack yelled, "I don't want to read that fucking book!" Mortified, I turned slowly to see if anyone had overheard his little outburst. The man seated directly behind me said, "Hey, don't look at me---I didn't teach him to say that."
It's way too late for my husband's cousins to become more prudent with their speech, and for me and my son as well, but I offer this cautionary tale to help anyone with little kids or grandchildren. Watch it.