I agree with Kathy, you need to be in control and you need to be silly. You need to interact with the kids and guide them to things you want....example: "are you a big sister now? I bet you are an awesome big sissy...can you give your sweet baby a kiss on the nose?".
Tell them what to do! Go tell mommy a secret! Oh wow, you see those flowers over there? Can you pick one and give it to daddy? I also make a lot of 'wrong' jokes...."hey, what does a cow say....do they say arf arf??" kids love stuff like that and you can get some real genuine smiles!
here I said "can you pick a pretty rock and hold it up so baby can 'see' it"?
this toddler would NOT cooperate...until I suggested getting on daddy's shoulders
I am not one who cares about smiles. Sometimes I just blab about a ton of different subjects until they hear something they like...glance my way, and when they make eye contact I am ready.
I'm no pro but I hope these images help!