Do you know where the older child was buried? It's unlikely the baby would have had her own grave, so maybe she is with her brother and possibly that stone may have a reference to her.
After your kind reply to my query, I looked at Ancestry to check again that there was no link to an image of the grave of the older child (John William Richardson, born and died in 1855). That is how I've already found the image of the grave of his parents, Harriet[t] and John William. But there is no image for their son's grave. I know from the parish register that all three were buried in the graveyard of St Andrews in their home village, Willoughton.
The 'Find a grave' website lists 245 Willoughton graves at St Andrews, but none is for John William Richardson who died in 1855. There was another family called Richardson in Willoughton (including a John William!), who became very small-scale farmers and who have family members buried in the 20th century in Willoughton churchyard. But no link has been found by me, in Censuses or parish registers, between this family and the far richer John William Richardson, whose daughter's birth was advertised in 1862. The 'other' Richardsons are not mentioned in John William's brief will of 1868 or in Harriet[t]'s extremely detailed will in 1871.
Harriet and John William are buried in one grave very close to the Church, to the right of the path as you approach the main door - near a grand railed-off tomb (occupant unknown to me). Clearly this was regarded as a prestigious burial place. Perhaps their baptised child, at least, had been buried in that area.
Harriet[t] survived her husband, so presumably could have chosen his place of burial in 1869- and perhaps could have chosen to use the spot where her baptised son, John William, had been buried in 1855. But she may have been ill - she died of Bright's disease (of the kidneys), 4 years after her husband. She moved remarkably quickly from her husband's big farmhouse into a specially built smaller house, which must have meant a lot of planning & upheaval. Her husband's death was not registered (by a servant) until 3 months after his death date, which suggests Harriet[t] may have been very busy or pre-occupied. It is not clear if the gravestone was erected after John William's or Harriet[t]'s death, or whether the person who arranged it (possibly Harriet[t]'s niece) knew the whereabouts of Harriet[t]'s son's grave.
Willoughton is 150 miles away, and, for various reasons, I think I'm unlikely to be visiting it soon. I may be able to find someone to check the older gravestones for me, just in case... Meanwhile, thank you very much for your help!