Children Love to Move and That's the Beauty
The amount of time that a child will stay on task is not very long so, therefore, letting them have more freedom during a session is great. I like to get my subjects in place and then just let them do their thing because those poses are just the cutest and most natural. By doing it this way you allow the child to show their most unique personality and letting them incorporate some of their own unique movements.
Kids love showing you who they are so why not let them. These can become some of your most treasured shots and pictures that your children will love to have and show to their children. In the future when their children ask, "What did you look like as a kid"?. What things did you like to do?
It is like providing your children with a story which they love.