I've been trying to explain to Nico and Lucas that there is no such thing as bad words. Words are just words, after all. What makes them good or bad is the intent behind the words.
Nico didn't understand, so I told him to think about how the words make other people feel. When you speak to someone, are trying to make the other person feel good or bad? What reaction did the other person actually have? Sometimes the person you are speaking with may think you are saying something bad, when you are not trying to. Miscommunications can lead to hurt feelings and arguments.
So long as the boys are being nice and respectful, I do not care what words they choose to use.