why cannot say 'shut up'?

The kids were talking about anything and everything under the sun as usual in the car when I was sending them to their grandparent's place after I picked them up from the childcare centre. Out of nowhere, Kai Kai told Ying Ying to "shut up", and Ying Ying quickly followed with another "shut up".

I interrupted their conversation instantly and said, "Kai Kai, you should not say that. Asking someone to 'shut up' is very rude."

"But uncle, why is saying 'shut up' very rude?"

I know the kids were waiting for my answer but I just could not think of a most appropriate response immediately. I said, "If you think that 'mei mei' is speaking too loudly, you can ask her to speak softer. If you want to tell her something but she is talking, you can ask her to listen to you or wait for her to finish talking."

"But my teacher also say 'shut up'. How come big people can say 'shut up' but children cannot?"

All my eighteen years of schooling and twelve years of teaching has not prepared me for questions like this. The best I could come up with was a lame excuse for the adult.

"Your teacher made a mistake. Next time she says 'shut up', you can be a nice boy and tell her that Uncle says it is impolite to say 'shut up'."

To "recapitulate" the "lesson" before I drop them off, I asked 4-year-old Kai Kai, "Instead of asking people to 'shut up', what should we say?" My smart nephew turned to his 3-year-old sister and said, "We should say 'please speak softly' or 'please listen to me'." Ying Ying nodded and said, "Orh..."

As I drove home, I began to worry if his teacher would jump and smack him for being rude and arrogant when he corrects her in class. Did I just get my little nephew into trouble?