I had a father of a newborn say to me during a newborn photography session, "If you think about it, babies have only been alive for a few weeks, so every little thing that happens to them is literally the worst thing to happen in their whole lives. That's why they cry all the time." It was the perfect analogy? metaphor? verbalization? of a belief I've held for a long time. It's why I run my sessions differently.
Kids are just like adults. They are nervous about having their photos done in the first place. On top of that, a stranger (who is hiding behind a black box for most of the time) is following them around and their parents are stressed and worried. No matter how old you are, when your parents are stressed and worried it makes you uncomfortable. They have ALL the life experience. They are your cue that something is wrong. Sometimes even too much excitement can trigger a child into the "oh god, why is everyone yelling at me face." haha. It's a careful balance of moving the dial up and down based on how kids are reacting.
This is why I like to do an activity during our sessions. Going for walks or playing at a park can help break up the "stand still and smile" monotony of photos. It's no longer a "performance". It's just playing. It's also why sometimes I'll put down the camera all together. A stranger behind a box is scary, but an adult who gets down on your level and takes an interest in that particularly cool rock that you just found is your friend...and smiles come easily around friends.