In the 1980s, Jeri Jaeger, another linguist mother, decided to write a book on children's slips of the tongue after a colleague informed her of the "well-known fact" that children didn't make slips of the tongue until age 7. She subsequently sent him a list of 100 slips that her daughter had made before the age of 3. Kids don't make slips of the tongue very often, but if you're around kids all the time (because they live in your house), and pay special attention to their language (because you're a linguist), you'll find that there's a lot of territory between "not often" and "never."
Parent-child studies helped popularize the use of empirical research in linguistics; they have inspired new theories and exposed facts about language behavior that no one had yet considered. They have, in brief, been good for the profession.
They also don't seem to have done any harm to children. The point of these studies is to describe nature, and so nature is allowed to take its course—it's just being observed and documented more closely than it might otherwise be. There is still, of course, the potential hazard of exposing intimate details of a child's life that he or she might not like to have exposed. It's lucky that the obscure 1919 study "Parallel learning curves of an infant in vocabulary and in voluntary control of the bladder" was made in the pre-video era. For reasons that they don't explain very well, the parents of the little girl in this study thought it would be interesting to compare her toilet-training success rate with her rate of word learning. So they logged over 4,600 entries on her potty "successes" and "accidents." (Their approach didn't have much of an impact on science. In retrospect, it looks more like a harbinger of the Facebook status update.) But as long as scientist parents follow professional guidelines about subject privacy and leave highly personal information out of their studies, kids are more likely to feel violated by their parents' YouTube accounts than by their journal articles.
The child's potential embarrassment is not the true issue here, though. Often the real objection arises from a harder-to-explain feeling that there is something unfair, or just cold, and a little icky, about a parent turning the microscope on his or her own child. The child has no choice; the parent has all the power.
Ironically, however, Roy's study indicates that the balance of power could shift as technology becomes more sophisticated. With Roy's Speechome project, it's possible to analyze not just what the child is doing, but what he hears—which is to say, what Roy and his wife are doing. At the recent conference of the Cognitive Science Society, Roy's team presented a paper that turned the microscope on the parents, showing how the way they alter the complexity and prosody of their speech influences the way the child learns. In an e-mail, Roy told me that he set out thinking that "language development" described a process that the child went through, but in analyzing the data he came to see it as a process that the parents go through as well. The more all-encompassing and detailed the study of child language becomes, the more we end up looking at what goes on around the child. The parent becomes a subject of his own study.