I've been asked by many people how do you get kids to smile. Well Unfortunatly I can't get all kids to smile. There is a such thing as a serious child. For the most part if you just talk to the kids and joke around you're bound to get a natural smile. For my oldest son, he loves to play the 1 2 3 game. The 1 2 3 game is where I count (very slowly) to 3 and after I say 3, he gets a good tickle. I get natural smiles and great expressions that way.
For kids kids 3-5 you can try the Repeat After Me game. They have to repeat everything you say, and then you toss in a silly word that they may have trouble saying. You'll be rewarded with a great natural smile, giggle, or a cool pensive expression.
Remember a great picture doesn't always have to have a smile. A great picture is all about capturing an image that speaks to you; a picture you will love for years to come.