If the photographer had to peer through the back of the camera to frame his subject, it is understandable that he would want to lift the subject rather than lower the camera. However, he and the parents must have had confidence that the child would make no sudden moves. Perhaps the convolutions in the curtain behind conceal some cords constraining the chair! A conservative design on the back for the early 1870s.
The design on the back of the card below is typical of the later 1870s.