The "hidden mother" photographs appeared shortly after the birth of photography in the mid 19th century. At that time the exposure time of the camera was very long and could easily exceed the patience of a child. This was a problem because even the smallest movement could result in a blurry photo. One trick used was to cover the mother with a blanket and place the child in her lap. The mother could then hold the child steadily and perhaps whisper threats (or promises of candy after the session) in the little rascals ears. Usually the photo was put under an oval mat which would leave only the child visible.
Tintype from the Hector Meinhof Hidden Mother Collection