Children are magical creatures. When they are left to their own devices, they can be silly, quiet, calm, animated, smiling, frowning, and laughing, but the second your attention is on them, they freeze up and become stiff as boards. You may as well build a bookcase out of them as take a photograph of them.
Allowing children to be their natural selves requires someone who can get down on their level and literally become one of them. A childlike joyful approach to living in the moment is best and allows children to show their silly or somber side. Either way, it will be the child you know. The one you observe when they are in their own little world and the smiles and laughter are genuine products of the instant.