It is pretty obvious -- but 'hidden mothers' photographs are called that because there are (largely unsuccessful) attempts made to hide the mother by hiding her with various props, so she would blend into the background. I don't know if the photographs were meant to be left like that or if it was intended that they would be cut down to size at a later date to effectively cut-out the badly hidden adult.
There are various reason for an adult being there in the first place: the time to take a photograph was a lot longer back then, so adults were needed to control fidgeting children, infants needed to be propped up to face the camera and -- sadly -- some of these type of photographs were taken post-mortem, so it was actually a dead baby or child which was being propped up for a memento mori photograph (though I don't think the latter was the case in the above photograph -- or at least I hope not).
