Yesterday in a discussion of painted portraits, especially those of children, an artist friend commented on the seemingly FORCED formality imposed by artists or parents on their otherwise rambunctious subjects. Interestingly enough, just a day or so before, in the check-out line of the local supermarket, I'd had a potential customer comment that HER brood wouldn't sit still long enough for a portrait. My wife I overheard inform her, "he takes pictures of them." She commented, "Not my crew, he'd have to run to keep up with them." I laughed and told her, "I can run pretty fast." I can't, but she didn't have to know that.
Speaking only for myself, I think the traditonal, neatly groomed, propperly behaved poses in question are at least somewhat market driven. Those with the "big bucks" needed to commission artists whose fees customarily run into the THOUSANDS of dollars, are often, by their very nature, somewhat formal, conservative, traditional people with a traditional home decor. Thus they expect a fairly formal, highly disciplined pose. Moreover, they have seen this type of portrait at the homes of their friends and want to imitate it. Also, there is a tendency amongst adults to think of their upper-class children as "little adults" ar at least they want to picture for posterity and their friends as having perfect "little angels."
Now, having said thart, I'm sure there are exceptions, perhaps numerous ones. In a more informal setting, portraits of children at play are maybe more common than one might guess, simply because artists may not "put up" this type of portrait as examples of their work, either because it is more difficult to do and thus the results may not please them as much as the others, or simply for reasons mentioned above, it's what their upper-class clients EXPECT. Another factor that may come into play is that the more "action" in a painting, the more it begins to remind the viewer of a photograph and often portriat artists, if not their clients, are acutely aware of this phenomena and try to avoid it like the plague. While they may often USE photographs in their work, most portrait artist would cringe at the word "photographic" as used to describe their paintings. It's also one of the reasons you don't often see broad, toothy smiles in professionally painted portraits. It gives the face a "frozen" quality. Despite this, I often encourage my own subjects to smile (not too broadly perhaps) but pleasantly, and in general, I totally agree that kids should be kids, even at a thousand bucks a shot. One of the favorite portrait I ever did was of my son at age four riding his tricycle, and I often use it as a sample. I've NEVER had a request from a parent for that kind of pose though.