Probably the #1 question I get from people is, "How do you get everyone to smile?" For starters: I don't. As I say on my web site, "I'm not so interested in capturing the physical features of someone for posterity's sake. My goal is to draw people out, get them to relax and laugh, and capture a glimpse..." Many of my photos include people with eyes closed, because they're laughing so hard, or include one person looking at someone else, because I love the interaction you can see there, etc.
That being said, I do always try to catch at least a few images with everyone looking and smiling. I can never state enough that I'm no expert--but from the land of "what I think I know", here are my top five ideas for how to get that "everyone looking and smiling" shot:
1) Make time to endear yourself to your subject(s).For example, just as I was attempting to get this shot of Matt and Gena...

...I was ambushed.
Not once...

Not twice...

...but three times:

I wasn't irritated. I was
thrilled! When Chase asked if they could mimic the characters in the mural...

...I was even
more thrilled! I kicked Ma and Pa to the curb as fast as I could! When the kids are into it, and having fun, I'm a whole lot more likely to get those "everyone look and smile" shots when I ask for them. With every family shoot I can remember, I have taken several shots that I had no interest in getting, compositionally speaking. But if I give my subjects the time of day, they're much more likely to return the favor! ...so that's tip #1. When your subject(s) suggests something you didn't have in mind, oblige them! And smile inwardly. Because that's a great sign that you're developing that essential repoire with them!
2) Be assertive with parents, and share some guidelines up front.Honestly, I often forget this one myself. When I remember, I like to encourage parents to leave it to me to get their kids to look and smile, etc. Because sometimes, well-meaning parents will try to help by snapping their fingers in front of their kids' faces, or giving them a little "pep talk", or what not... which results in lots of pictures with parents' snapping fingers in front of their kids' faces, and lots of not-so peppy expressions... ;0)
3) Whatever you do, do not have them say "cheese".Say cheese. Yes, you. You who are reading this. Say cheese right now. Out loud. Please? It makes me giggle just imagining you sitting in front of your computer saying cheese... but I digress! Freeze your "cheese", and look in the mirror. Go ahead. Just do it! I'll wait..................... Okay. Are you back? Not so pretty, huh? Saying, "cheese" pulls your mouth into a tight, awkward position that is not really even a smile. My sweet Kendra was kind enough to demonstrate for me:

You gotta love how even her arm muscles are in full flex. She puts her whole body into a "cheese" grin! He he.
I'd take a silly face, a smiling face not looking into the camera... anything over an unnatural "cheese" smile, any day:

Ah, much better, to my way of thinking.
4) Make them laugh.So what do you have them say, if not, "cheese"? How about "supercalifragilisticexpialidocious"? Or, "We love to eat bricks with cheese on top!" I try to think of the most zany things I can. Because I'm not really looking for a smile, after all. I'm looking for a laugh. I think when someone is laughing, they've got their most beautiful, natural smile coming through. They can't "pose" their smile when they're laughing. I do all kinds of crazy things to get people to laugh. Mostly, I'm just an idiot. One of my subjects once said to me, "You know we're laughing
at you, right?" ;0) I know, I know! I'll do whatever it takes!
5) Get Grandma, or a favorite Uncle, etc., to come along.I owe MANY of the "everyone looking and smiling" pictures I've captured to the Grandma, Uncle, sister, friend, or whomever standing behind me. When I do a session, I like to have FUN, and I like my subjects to have FUN, too! We dance, we sing... you name it, we do it. ;0)
I hope this might give you a few ideas for your own attempts at "everyone smiling and looking" shots! Have a supercalifragilisticexpialidocious time trying to get them!! ;0)