In the name of self-expression, we let a lot of things fly that in common
decency should not. A standard has to be drawn somewhere because kids are
first forming theirs, and they need to learn what is acceptable and think of
respectable alternative ways to express their thoughts. The child's
illustration says a lot in itself. The words were not necessary. I have a
feeling you were "had" because he sensed your hesitancy.
I tell my kids that if they want to be treated with respect (which many say
they do not get because they "didn't get their way," right or wrong) then
they need to conduct themselves with respect so they EARN it from others.
That does not include inappropriate language. I usually send home a form I
generated where they write EXACTLY what they said and then rephrase it into
more acceptable words and have the parents sign it. I find they do not use
this language with their parents and are shocked!