“I have a four-year-old son. He goes to preschool, he often meets his friends in the yard. He has been using bad vocabulary for some time. I am afraid of the opinion of the preschoolers, which are inclined to suppose, that we apparently use such words at home. What to do?”
It seems to me, that there is no need to worry. I assume, that the kindergarten staff has pedagogical knowledge and is oriented, that three-, four-year-old children go through a phase of fascination with such words. They interact with their peers, learn other vulgar terms and use them with pleasure. In this way they try to impress their colleagues with courage (they know after all, that they are doing wrong). They also show off in front of adults, especially when they know, that you would be able to shock them this way. So I advise against violent reaction. It only worsens the situation. Some kids keep swearing even at home, counting on it, that they will upset parents. They are proud of it, that they manage to scandalize the environment. Others, under the threat of punishment, fall silent at home. However, I do not recommend not reacting. I advise you to make a harsh statement, that neither you, nor do other adults wish to hear such words.