This baby seems to be in absolute terror of the photographic process! The photographer has used an oval vignette mask for this tintype. If his purpose was to hide the mother it was not entirely successful. Her collar, chin, and a bit of hair can still be seen. At first I assumed the wavy line at the bottom of the picture was part of the baby's dress but an examination at a higher magnification shows that the polka dot fabric ends at the white patch that might be a sock. I think it is a ruffle on the mother's dress and the wavy white line is the same binding that is on her collar.
It is part of a lot of tintypes that were described as "Hidden Mothers" by the seller. They are all wonderful and I will post them all but am starting with the five where I can clearly see a part of the parent. The others might be but it is not as clear to me. Sometimes a chair covered by a blanket is just a chair covered by a blanket.