Have you tried saying "Cheese" and it doesn't work? They stare at you grumpily or give you that fake smile that looks like a grimace? Let me share with you a trick I learned from my mother. Tell them to NOT smile. Really.
It goes something like this (speak like you are serious but be smiling a little yourself):
"DON'T you smile!"
The child looks surprised
"I mean it!"
Small smile starts
"I see that! Wipe that smile off your face RIGHT NOW!!"
Big open grins and belly laughs
Keep talking as long as you are taking pictures, with variations on the theme. "Don't you dare" does wonders, too.
Since daddy has been doing a lot of business in Seattle he needed to demonstrate how far it is from Reno to Seattle.