1) No Posing
First, remove the phrase "cheese" from your vocabulary. The moment a child hears this phrase, they are willing to put on their "picture-face". Try to catch them in the moment when they are laughing or being silly. The expressions they give you are most genuine during these moments. One thing to remember, it is not always about getting them to smile. A child reveals their personality, energy, and uniqueness while in a natural state. (Candid photos are the best!)
2) Getting Comfortable
You, the photographer, are a stranger to the child. Before you begin your photo