One thing I've learned about photographing people is to keep clicking past that first or second shot. This is especially true with kids.
I will demonstrate:
Camera ready, I say "Hey, look over here."
And I get this. So I say, "Can you smile?"
So he smiles. Cheese-ball smile. So I say, "No, really smile!"
So he does cheese-ball to the 4th power. So I say "Just smile NORMAL."
So he frowns, of course. So I tell him to say pickle, and he says pickle like an angry sailor would say - well nevermind. So I say, "Geez, are you mad at the pickle?!"
And -- he giggles.
And finally, he looks at me and smiles like a normal person.
When he smiles at me I swear my heart explodes every time. (It's possible I'm not going to be a very good mother-in-law.)