I find that it's much easier not to attempt to "pose" a child, but rather allow him or her to explore and do what they normally would do. This is the least frustrating and most stress free for the child.
This page is an example of lots of patience and simply waiting for the right moment to capture a portrait.
Kids are wonderful, until, you try to schedule or direct their expressions/attitudes/poses.
If you schedule a portrait session with your child, please allow yourself plenty of time for things to "happen" on their own.
The very instant the shutter closes and the image is stored...
What is captured, is the past.
A photo will never forget and fails to judge or discriminate against the subject
As you are now, in this moment, you will never be again...