The younger the child, the messier the face (usually). You could have just shot the golden image, only to discover a face-full of crumblies and cracklies in post processing. Doh! If you would have just remembered to wash his/her face BEFORE shooting, you would have saved yourself a ginormous amount of time. Same with hair — bring a small spray bottle and comb to tame any crazy, untamable hair. Remember to let the hair dry, unless you want the wet look. Kids need styling just like adults, and are much more likely to need constant touch-ups. If the hair isn't cooperating, reach into your bag for a hat or cap.
Kids move. And move again. Again. You can set up the tripod and manual focus for a couple of minutes, and then they move. You have to re-setup. Then you have lost two minutes of their short attention/boredom span. Just set the lens to autofocus (I use autofocus lock) and shoot away. Pause every once in a while to see if you are getting what you want, but don't be afraid to shoot many frames. If you are shooting outdoors, you can get away with holding your camera. With younger children, prop them on a fence or a chair to immobilize them in front of your desired background. This will prevent them from running away easily and it will be easier to get them in focus.
Yes, children are cutest when they smile. Their adolescent teeth or lack of teeth can be great. But some of the most captivating photography is capturing an emotion other than a smile. A serious face can show more personality than a smile or toothy grin. Of course the smile one is nice and sweet and needed, but after you get that perfect smile, go for a conceptual grumpy face, a game face, or a make believe mad face. These can also be hilarious. The second face beyond the smile will say more in the end.
child's sense of imagination can be easily tapped into, and will almost always bring some hilarious and unexpected results. If you have the child in costume or theme, let them act the part. The swashbuckling bard! The kid doctor checking for a heartbeat. If they are dressed up for a sport, have them celebrate winning a championship. Imagine with them. They may just bring out that crazy, 'never-would-have thought-of-that' concept for you.
The younger the child, the more patience required. Their attention spans are not very long, and they get bored and distracted easily. If you have multiple child models, it may help to switch them out if one is not cooperating to your ideas. This can sometime help them settle down and want to try to model again. Most of all, you just have to take a few extra deep breaths, and maybe improvise. Be careful to never push a child too far, or you may have to reschedule the shoot.
Don't forget the incentive! Even if you are getting paid for your session by the parents, the kids need to be shown some love as well. Small treats are great, and get their attention. Maybe it is a pack of sports cards for the boys, and a new doll or accessory for the girl. Of course, this depends on age, but if you show interest in them, they will usually respond back. You don't have to go over the top and buy them a video game or a Wii, but show them the concept of work and reward and you will have an easier time getting them to cooperate. If the session does not work out and you promised the child(ren) a reward, don't punish them by not rewarding them, or they may never smile in front of the camera again. (Incentive may be a good idea for the adult parents too.)
Don't be afraid to try something new if nothing works. You may need to eat an incentive as well. If your child model is being sad and grumpy, get some sad and grumpy photos. The girl is giggly? Then giggly photos are needed. Get what you can out of the situation.
