It is normal for children to by shy especially in front of the camera. Usually they need to be distracted with a toy or food (candy works best) to keep the attention away from the lens. Here the photographer took advantage of her niece playing hide under the table and to the girl it is now a game.
Another way to handle this problem is to give the model something to do. Here the boy is preoccupied with writting on the ground. What a great idea.