I know, it seems impossible. A lot of times it is. Here are some tips:
-Tell them all to look right into your lens and depending on their ages and gullibility, tell them that a tiny blue bird lives in there and if he's awake and if everyone is looking right at the camera and smiling, he will pop out and sing a song. OR tell them you think you might have a problem with your camera and ask them to look and see if a red light blinks inside the lens when you take the photo. I have gotten some REALLY funny pictures this way, unfortunately none of my own kids or I'd share, but I've gotten some interesting, intense little faces.
-Tell them to look serious and teasingly forbid them smile (Don't you smile, don't you do it). Kids CANNOT do this. I don't know what it is but it cracks them up every time. When I'm getting the big time cheese grin, I say, "Show me your best sad face" click, "Show me your best surprised face" click, "Show me your best mad face" click, "Show me your best silly face" click, "Show me your best serious face" click, "Show me your best happy face" click. I almost always get a genuine smile somewhere in all of that and I usually get a full range of other hysterical expressions as well. Together these make for a great grid or storyboard.
-This is so annoying, but in this digital age, the little brats want to see themselves immediately. Often I tell them we're going to use a magic spell that I got from the Fairy Godmother, and we're going to say the magic words, and then we're going to do our very best smiles and see if we can take the perfect picture. Then take the picture, everyone look at it, and decide if you need to try another spell.
At the studio I'm able to hook my camera up to a television monitor, but I always disconnect it if I'm doing kids because they will be completely unable to tear their eyes away from it if I leave it on. Vain little creatures.
-I have come to realize that it is just impossible for both of my children to be cute and cooperative at the same time. So now when I'm planning a photo, I try to think of something that won't require it. For instance, when we go to Savannah in May, I'm going to get a photo of them in overalls and no shirt, holding hands (hopefully) and walking away from me on railroad tracks (no trains on the day I'll do it, don't worry).
Recently I set up a tea party for two sisters, on a pretty little table, with them in Easter hats and with china teacups and a teapot and a vase of flowers. These two were anything but cooperative but the ONLY shot I got of them sitting at the table was perfect. Only because it required nothing of them other than being there.
So if all else fails, and no one is cooperating, tell them to turn around. Sometimes pictures of the back of them can be just as cute and you'll LOL remembering why you had to take it that way.
Also, if they're your own children, consider telling a friend to take the camera and you can do the same for her kids. Because kids just don't behave for their mother when she really wants something.