For the vast majority of portrait painters who paint children, the keys to success lie in one of two areas, vast patience of Godlike proportions, or a good photographer...preferably the artist himself/herself. Children come in all sizes, ages, and dispositions, with attention spans of up to fifteen minutes--plenty of time for an adept photographer--not even CLOSE for most portrait painters. Yet despite this, and their penchant for flash photography, I've often found parents make the best child photographers for the same reason they often kill their children (or feel like it)--motive and opportunity. But allowing for the fact they may not all be too good with the vagaries of cameras, films, and lighting, the artist can often fill in with far less danger to the children themselves. And, the artist has the added advantage of knowing something about various lighting and compositional rules which are consistent between photography and painting. Also, he or she knows what's needed in the way of pictorial source material, given their level painting competence. That is, the better the painter, the less important is the photograph.
Though I'm far from an expert, I've done a little photography and I've done a little portrait painting of quite a few little kids. Also, I've learned a lot from a book entitled "Creative Techniques for Photographing Children" by Vik Orenstein (Writer's Digest Books, $24.95). Vik (not Vicki) sells photo-portraits of kids. She has to be good. I only paint from them, I can be less so. But the key factor here is the same. We both MAKE pictures, we don't TAKE pictures. It may seem like a trivial difference, but it's not. It's a difference in attitude. It's very much like photographing artwork. You think of the CHILD as the work of art. You, as the artist/photographer, are just depicting a fun-loving young masterpiece. And despite what I said earlier about their attention spans, you really do have a lot going for you.
Almost without exception, kids are cute, or can be if handled right. And those who aren't, can at least SEEM that way at some point between the snap of the shutter and the last stroke on the canvas. There was a time when painted portraits of children had about as much life as the stiff-backed chairs upon which they were posed. Take a look at nineteenth century child portraits sometime. In large part, photography was responsible for changing that. When painters began losing portrait commissions to photographers, they quickly came to the realization that, hey, painters can photograph too...and they did...and in the process, a marvelous change came over children's painted portraits. They came alive. And that is the all-important trick to painting good child portraits--making them breathe.
Now, since everyone is waiting breathlessly for a few concrete tips...next time you point a loaded camera at a kid, try this. First, get rid of the tripod. It inhibits both the photographer and the photorgaphee. Second, interact as a friend, suggest but don't direct (for the same reason). Be ready for anything, think on your feet, but don't think about how much film you're shooting, think composition, think lighting, think angles, better yet, do all of the above WITHOUT thinking...make all these things instinctive. Remember, as a painter, you're NOT selling photographs, so you can do things to your final images photographers (even with computers), only WISH they could do. Be watchful...look for moments when children's personalities reveal themselves in posture, expressions, and mannerisms. Have an itchy trigger-finger--better to shoot a dozen bad shots than miss one GREAT shot.
And this may sound radical to some, but I've always had good luck shooting in black and white as opposed to color. What you need is an image. What you DON'T need is to be influenced by what is often poor or at best mediocre photographic color. Leave that to the experts. As an artist, YOUR color sense should be better than that of the film, and in any case, you have the real thing to refer to, however briefly, under ideal lighting conditions if you so desire. And finally, as a painter, unlike a photographer, you don't need ONE good picture. You have the luxury of being able to work from a DOZEN mediocre ones with perhaps just ONE pretty good one if you must. You know, I never realized until now, how much easier portrait PAINTERS have it over portrait PHOTOGRAPHERS! We have so much more TIME in which to achieve perfection.