We have all been there. We have been in a store when a child starts crying or begging or whining. Hell, it doesn't have to be a child. Even to this day when I go to the grocery store with my mom, I can throw a pretty good tantrum if she decides to try and buy something green and leafy.
Normally though I think are thoughts are not so much as why can't that parent get the kid to be quiet, but more sympathy, because there are so few options a parent has to deal with a crying child. We don't know why they are crying. It might not be because they couldn't have the candy or toy or latest Jonas Brothers trading cards. It could be because they are sick or just had shots or really tired.
You just never know. I do know though that I wouldn't deal with it by walking up to the kid and slapping them. Apparently though Roger Stephens doesn't live in the same world you and I do, because he was in a Georgia Wal-Mart when a two year girl wouldn't stop crying. Stephens who is 61 and looks like the crankiest a-hole ever walked up to the kid and said to the mother of the child that if she wouldn't stop the child from crying he would make her stop. Well, she kept crying and he slapped the girl four or five times and then said, "See, I told you I would get her to shut up."
He was then arrested and charged with first degree cruelty to children.