all of the above.

You are a twenty-three-year-old, single computer programmer taking your ten-year-old niece out to lunch at Taco Bell after a morning of high-yield yard saling. She sits down with her food while you stand up next to the table, determining if you have enough napkins, silverware, and other eating accessories. Your niece shoots you in the chest with her straw paper. Do you:

A.) Sigh disapprovingly and look unamused without saying anything, thereby putting a tension in the air, which then causes the entire meal to be conducted in awkward silence. She will know her misdeed and not commit such silly acts anymore without you having to say a word.
B.) Start lecturing her about not acting like a child, at least not in public. Be sure to stress the importance of growing up and emphasize the advantages to increased responsibility so that you do not come off as too strict.
C.) Ignore her with the understanding that giving attention to such acts only increases their frequency.
D.) Smile at her and dismiss the incident, showing that you are an adult with more important things to worry about than lecturing her over an innocent prank, but that you have not forgotten your own carefree childhood. Kids will be kids.
E.) Shoot back.

Option E. Duh.

My straw paper hit her directly in the eye, and stuck in her glasses. I felt like the biggest failure as an adult ever, that I could not even take a ten-year-old to Taco Bell without directly causing her bodily harm. But Social Services didn't swarm upon me, and no one started giving me option B. We had a pleasant lunch talking about my job, the concept of compromise, and the difference between circus clowns and hippies, and she seemed to be without emotional or physical scarring. We left happy and full, a girl and her aunt, or a girl and her niece, depending on how you look at it.