Everyone has pretty much covered it !
I had the same problem for quite a while but you just have to take the chance and try some things out, you'll soon discover what works for you and what doesn't.
Adults and older kids get a kick out of you acting like a soldier or a cop around them, asking them if they have a permit to film, if they have noticed any droids, and that kinda thing.
Because smaller kids are usually quite timid normally I'll just say hello, ask them what their name is and shake their hand or give them a high five, you just wanna try and appear as unthreatening as possible towards them.
Unless they assume the attack position with a toy blaster, then you can have some fun with it
Just act the way you would imagine a TK would in any given situation and you wont go far wrong.
And most importantly, have fun !
Its amazing how willing the public can be to go along with the act as long as you don't force it.