Bribery isn't always bad.  When trying to take group pictures or an individual holiday portrait where small children are involved, you are much more likely to get a good result by keeping the mood light and offering a reward for good behavior if you need an additional incentive.  Children who have been punished for not smiling (or fear a punishment shortly) have a hard time relaxing and producing the expression you are hoping for.  Likewise, if you are stressed out trying to get the perfect picture, they will sense your stress and it will be reflected in their faces.  Enjoy the moment, offer a special treat at the end and capture who they are today - whether it's a six year old with a "cheesy" smile, a four year old who wants all the spot light or a two year old who would rather be photographed in mom's lap.  Next year, they will be different and you will enjoy seeing who they used to be.