When dealing with children, you're better off breaking the ice with a wave and waiting for them to come to you. You can offer an outstretched hand, but be patient. If you bum rush them you'll frighten them. I actually had a child psychologist come up to me and complement me on this technique at a recent event.
Watch your posture. You want to stand straight and tall like a soldier and never lean. Find a comfortable stance. If you're going to walk around, survey the crowd like a police officer.
For some reason lots of people seem to enjoy having their pics taken as if they were under arrest. They also like having blasters pointed at them. I often bust people who think I don't notice them taking a pic by pointing my BFG in their direction.
I also agree with a lot of what's been already stated. "Can I see your identification, sir (or ma'am)?" is gold as is asking how long someone's had their droid(s) while indicating their children. I also refer to everyone as "sir" or "ma'am".