… but I don’t think they are the mothers.
I’ve come across those images months ago, and then again recently: A small child, sitting on the lap of a shrouded woman. It usually looks eary, if not downright creepy in cases where the mother is hunching behind a chair, like this:
The mother hidden under a cloth is weird enough, and why not take a picture of mother and child? those images are common enough, and have been around even before photography.
But a mother, a woman wealthy enough to afford very fine clothing for her child as well as an expensive modern portrait, and she’s all but lurking, crouching behind a chair? Even worse, in some images the women in question have a rather rough cloth spread over their face and upper body ; it’s unpleasantly reminiscent of putting a paper bag over someone’s head.
So my theory is that this is a nanny, which would explain why it is thought necessary (or allowable!) to cover her up or make her crouch. Family portraits are an expensive commodity, and they are for the family, not for the servants.
But why have a servant there anyway?
Well, if you have seen just a few old pohotographs then you know that there are often blurred bits where someone, against all warning, moved a hand or even head. And if you took pictures of a small child you know how hard it is to get them to keep half-way still. From what I have seen, all those “hidden mother” pictures are of children who are in that age bracket where even with promises and threats, it’s very, very difficult to make them keep absolutely still for 20 or even 30 seconds.
My guess is: very often the only chance is to have the child on the lap of someone it’s very familiar with (the nanny, mother or maybe elder sibling) and who has experience in getting this small child, or small children in general, to behave well, at least for half a minute. Depending on how much time the mother spends with her child and how experienced the nurse is , putting the child onto the lap of its nurse or even wet-nurse may be the best hope for getting a picture of anything but a baby-shaped blurr. (And wet-nurses are not picked for their photogenic qualities.)