How to Photograph Children
Tip 1 - Don't Act Your Age
I'm a firm believer that if you want to take great photographs of children you need to act like one. To me, this means that you have to throw out a lot of your ideas about how they should look, how they should smile, and what they should be wearing. You need to let them do their own thing. If you create a photographic environment where they feel safe and free, you will be amazed at the moments and scenes that appear before you and your camera.
Dress, Smiles, and Poses
With that said, you may still want to pay attention to ensure that the children you are photographing have clean faces, unstained shirts, and combed hair. That's perfectly understandable. What I would recommend is that you do the combing and wiping separately from the photo session. All of that cleaning and rubbing can be uncomfortable for children and will put them in a mood that won't allow for relaxed photographs (you remember it happening to you, right?). Do the wiping first, then give them a few minutes to unwind before getting out the camera.
Let Them Loose
It's important that a photo shoot isn't seen by your young subjects as a chore or a job. You may need to have some rigidity in your location and poses, just to ensure you get usable photographs. But the more you keep those plans hidden in the background, the more you'll get from the kids. I like to make it feel like a free environment, even if it isn't.

I had just photographed this child (above) with his family. After a few shots, I gave the kids a break while the parents and I made small talk. In a minute or two we noticed that young Jonathan had crawled into the middle of these plants, with just his head popping over the top of them. The resulting photograph was a natural moment, and no posing was necessary. Would I have thought up the idea for Jonathan to crawl into the plants and peek out? I doubt it.
Cheese
One of the first things we teach our kids about photography is how to cheapen it. Wherever you go in the world, children have learned to make the "cheese" smile whenever a camera appears. As you can tell from my tone, I really don't like the cheese smile. Look again at the photo above of Jonathan in the plants. Picture his face making that cheese smile that we adults so often request. By not having it, the natural look we have instead is much more classic and long-lasting. Without the cheese smile, we are drawn into the entire scene, mood and moment, rather than to a toothy smile.
Do an experiment. The next time you photograph a child in a portrait situation, try not telling them what to do. That's right, don't give them any instruction other than something simple, like, "Stand over here." That's what I did when I photographed young Anna in a studio setting.
By not giving her any instruction I captured Anna the way she presents herself, rather than what I think she should look like. With no instruction from me, her personality controlled her appearance. If you intend to truly capture your subject and their personality in a photographic portrait, make sure your subjects' personality is exposed.