Telling a child to smile is to teach a child they do not have autonomy over their body. It’s simple. I don’t care if you want to take a photo. You’re also taking their independence.
I was one of those kids.
I grew up being known as the kid with a smile even though I was beaten every day at home.
My ex-boyfriend often grabbed me by the wrist. We had different opinions on what fun thing to do next on a Sunday. He decided that he was going to win that argument by physiccally moving me in that direction. I thought nothing of it.
My ex-girlfriend often accused me of various choices leading to hurting her feelings. I used to just stand there listening to her scream and yell for at least an hour, sometimes involving punching me, while noting to myself that she did that every week at 11 am on Saturday no matter what I did or didn’t do that week. I didn’t even realize that it was not normal to allow someone to steal hours of my life a week in this fashion and take away my autonomy over my physical presence there.
If I approach a cashier, who is a human being with feelings, I should try to be polite even at the deepest darkest moment of my life. A smile will have to be faked. By telling me to smile, you tell me to fake.
Do not tell a child, man, or woman to smile.