It's quite possible that the child may have been buried in the grave of another person. It was the custom for children to be buried in the same plot as another person buried on the same day, especially if the parent(s) had little or no money. If there were other family members interred in that graveyard, it's also possible that the child may have been interred with them. Either way, the existance of any marker would be fairly unlikely. When I visited a graveyard recently, the archivist looked up the burial plot records of my g.g. grandparents, and we found that their grand-daughter (who had died at the age of 4, about 5 years after they died).
