Photo graphing kids is a rather tricky ordeal for a portraitist but they are indeed marvelous subjects. A good way to start with is to stoop down to their level even if it would entail you to crouch or kneel down so that you could be seeing them eye to eye.
Kids are by nature, very unpre dictable coz they move a lot so you should be prepared to shoot in continuous mode to have greater chances of capturing more wonderful moments.
Letting them pose for you requires patience and demanding time and effort so photo graphing them when they're busy or preoccupied with something or while they're at play will bring more successful results and you'll be getting or capturing spontaneous moments or natural facial expressions instead of them thinking about you with the camera which may either result in them being seen with a frozen look or showing off for the shoot.
Shown above is Jovi whose smile slowly faded when it took me quite some time to adjust my camera's settings. The kid in red and blue shirt is Iggy. He seems to be frozen while staring at the camera which is pretty bad but fortunately, i liked the result here. The kid in orange shirt is Mattie. He is showing off while I'm taking the shot.