At first I wasn't so sure this could be a post-mortem photo as the woman doesn't seem to be wearing black, which is mandatory for the Victorian mourning customs. But after digging a bit, I find that according to Harper's Bazar magazine (an authority on fashion and etiquette), mourning for a child should last nine months and the clothing need not be completely black as for a husband, but should be edged with black crape. Since we can't really see much of the woman's dress, the crape edging may well be there.
Certainly the couple look even more somber than most photographic sitters, and the baby does not look at all normal.
Postmortem portraits of deceased infants were not considered gruesome in Victorian society, but were treasured mementoes of a departed family member.
As always, a sterling analysis from OR.
Cheers,
China