Children can be very awkward posers. Children go through a phase from about 3 to 6 years old where they are incapable of pulling off a natural smile - instead you get some weird grimace. Natural and candid shots of children are always the best.
Keep the children out of "photography" mode; give them a game or activity to play at. Something much more interesting that you and your camera. Set up the game in a way that gives you good lighting and bring the child to the game rather than the other way round.
Shoot quickly, emotions and moments are fleeting with children. Take a lot of shots, the more you shoot the "luckier" you will get and the child will become less aware of the camera.
You can't force a child to enjoy themselves. If the moment is not happening then move on.