Is it mean to back answer??

It was another sunny afternoon. I was in the park, as usual, with my son, playing. My son walked into this area where there are a couple of benches, some counting beads and some figures. There is this girl sitting in one of those benches, coloring a disposable plate. She may be a 2nd grader, or a 3rd grader perhaps (I am not good at guessing the age of kids here). My son and I started counting the beads and I was showing him the bus figure, pointing out to its wheels and started singing the nursery rhyme "The wheels on the bus", my son's favorite. Suddenly, this girl shouts at me "Will you keep quiet? I am not able to concentrate." I generally do not answer back to mean comments by strangers especially in public places... But today, I did not keep shut. I said, "Baby, this is a park and people will play, talk and sing. If you want silence, go to a library". I don't know why I made such a comment, that too to a small girl. I felt a little bad after saying that. May be if it was an adult, I might not have felt bad. If she had asked me to be quiet in a polite voice with a "please", I might have actually kept quite... But, none of this happened... I made a mean comment to a little school girl... Did I react the right way? Or did I over-react?